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1st gear, accelerate, sharply drop off the gas - and invariably get a heavy sharp knock somewhere in the bottom, middle to rear. Sometimes it is not one knock, but two or three. Sometimes the knock is delivered if I even gently ease the gas when approaching the curb to park. Doesn't happen on second gear and up (can't be sure, though, this is not my car, I just drove it around the block to diagnose the problem).

So, what does it sound like? I initially thought that was backfire (heard it when the car was driving away), but now that I drove it I would rather think that it's either the drive shaft lash, or, even worse, suspension or diff mount.

Any more clues?

I will have the car on the ramp as soon as I can, it would help if I knew what I should be looking for.

I'd look first at the differential mount. Try backing the car in reverse, a bad diff mount will be much more pronounced in reverse.

Coldswede wrote:I'd look first at the differential mount. Try backing the car in reverse, a bad diff mount will be much more pronounced in reverse.

Do you, by chance, mean part #33171125290 at http://www.realoem.com/bmw/showparts.do ... g=33&fg=10 ?

yes, that's the part he is refering to, although it can be had for much cheaper than listed on real-oem.

it's quite simple..5bolts to take off, and put back on, and makes a world of difference. As stated above, drive in reverse and see if it happens more, if it does, that's pointing towards diff mount, if not..let us know!

Dumb question - is the visual inspection of this part when the car is jacked up going to reveal anything?

(my hunch that it won't)

In case it doesn't, am I likely to see anything when I take this part off?

Something else to look at. My car was making the same noise in the same location. Turned out to be the exhaust (Bosal factory style aftermarket replacement) was hitting the body when I took off and then let off the gas. A sharp crack, crack, crack at about the parking brake area. I realigned the exhaust, put the transmission to exhaust bracket back on and the noise was gone. This may not be what yours is but is worth looking at.

Tony

Tony533i wrote:Something else to look at. My car was making the same noise in the same location. Turned out to be the exhaust (Bosal factory style aftermarket replacement) was hitting the body when I took off and then let off the gas. A sharp crack, crack, crack at about the parking brake area. I realigned the exhaust, put the transmission to exhaust bracket back on and the noise was gone. This may not be what yours is but is worth looking at.

Tony

I will look at that, too, but it seems to me that the sound is not a tin sound the exhaust would make, but a heavy, healthy knock of reasonably sized mass. With some luck, will see tonight.

I had the same problem described. Try the CSB (center support bearing). Bet it's toast. If you can address the problem soon enough, you can save yourself the expense of a driveshaft. Take a look at the transmission bushings while you are under there. If you have to change the CSB, may as well change the trans bushings while you are there. And vice-versa.

vt wrote:Dumb question - is the visual inspection of this part when the car is jacked up going to reveal anything?

(my hunch that it won't)

In case it doesn't, am I likely to see anything when I take this part off?

You might have to take it off to tell, but then it would be obvious, it will probably be in two pieces. Make sure you support the diff, but other than that it's just 5 bolts. Easy.

2ndeagle wrote:I had the same problem described. Try the CSB (center support bearing). Bet it's toast. If you can address the problem soon enough, you can save yourself the expense of a driveshaft. Take a look at the transmission bushings while you are under there. If you have to change the CSB, may as well change the trans bushings while you are there. And vice-versa.

Did you have vibration before you diagnosed this? This car doesn't have any at all - just the knock when the gas is dropped off.

If in fact your diff mount is bad, it will show itself when putting the car in reverse (if a stick, you'll have to accelerate in reverse, of course). A visual inspection will show that the rubber has pulled away from the metal frame. You want the M535i/528e mount at $70, rather than the $2xx 535 mount.

I was thinking center support, but it only seems to happen on deceleration, so there goes that.

wkohler wrote:If in fact your diff mount is bad, it will show itself when putting the car in reverse (if a stick, you'll have to accelerate in reverse, of course). A visual inspection will show that the rubber has pulled away from the metal frame. You want the M535i/528e mount at $70, rather than the $2xx 535 mount.

Do you know the part #, by chance?

I was thinking center support, but it only seems to happen on deceleration, so there goes that.

Well, it *does* happen only upon sharp deceleration - and thanks for giving me that clue, I haven't checked heavy braking, only dropping the gas. My guess would be that if it is the diff mount, braking won't produce anything - or will it?

Hate to be guessing, but didn't have a chance to take a second look at the car - it's still not here yet.

M535 diff mount 33 17 1 129 786 ~$62 when I bought one last but I don't remember where.

Braking won't produce this clunk, only engine brake decceleration or acceleration in reverse, same force on the differential actually. That thing is not so robust in the reverse accel direction and the clunk is probably from when it slaps back into place.

tn535i wrote:M535 diff mount 33 17 1 129 786 ~$62 when I bought one last but I don't remember where.

Not the first time I hear about replacement parts being better from 528e/M535i (last time it was the exhaust hanger ring, twice as thick) - defies common sense. More expensive and of lesser quality?

Try checking it in reverse, that should clue you in to CSB or diff mount to check first...eventually you're going to have to climb under the car and get dirty..

boardsportsrule wrote:eventually you're going to have to climb under the car and get dirty..

Oh, I have no problems with that (speaking of which, my other car is opened up right now, waiting for the new window regulator to come).

It's just that the car isn't here, I'm waiting for it to get into my hands so I can stop over-analyzing in vain and do something useful for a change. Waiting is the worst part.

What car do you have?

If it's not the center support bearing (and I agree with above about accell vs. decel) then the next thing I'd look at is the harmonic balancer. If your car has one - not sure about eta's.

California Dreamin' wrote:What car do you have?

If it's not the center support bearing (and I agree with above about accell vs. decel) then the next thing I'd look at is the harmonic balancer. If your car has one - not sure about eta's.

It's the DC74 production code 535i: http://www.realoem.com/bmw/partgrp.do?m ... spid=47231

vt wrote:

2ndeagle wrote:I had the same problem described. Try the CSB (center support bearing). Bet it's toast. If you can address the problem soon enough, you can save yourself the expense of a driveshaft. Take a look at the transmission bushings while you are under there. If you have to change the CSB, may as well change the trans bushings while you are there. And vice-versa.

Did you have vibration before you diagnosed this? This car doesn't have any at all - just the knock when the gas is dropped off.

My cue was the knock upon acceleration from 1st and diminished in 2nd and 3rd. I would often take off in 2nd gear, so that it would not knock. The knocking seemed to come from just behind the two front seats, which could be considered middle to rear, ( this is subjective, as the knocking will resonate across the floor).

2ndeagle wrote: I guess we cued in on different symptoms. My cue was the knock upon acceleration from 1st and diminished in 2nd and 3rd.

My fault, I should've been more clear. It's not acceleration, and maybe not even deceleration (still have to test that), but just sharply letting the gas go.

That sounds more like diff, if not attributable to accel or decel. You're going to have to crawl under there. You have the potential problem areas defined, the rest is visual and probing.

vt wrote:1st gear, accelerate, sharply drop off the gas -

Isn't that engine brake decceleration? Same as accelerating in reverse?

House(TV Doc) would say Diff Mount, it never acts up in forward acceleration.

CB/Driveshaft problems occur in either accel or deccel but tend to vary in the noise itself, similar for subframe bushings.

tn535i wrote:

vt wrote:1st gear, accelerate, sharply drop off the gas -

Isn't that engine brake decceleration? Same as accelerating in reverse?

All right, twist my arm.

House(TV Doc) would say Diff Mount, it never acts up in forward acceleration.

Just looked at it (by the way, it does give the same distinct knock in reverse ) - well, I don't know if the diff mount is the root cause of this specific problem or not, but it sure looks like it is mostly daed, a.k.a. borked. I'll bite the bullet and order the part.

Timeout till the part comes in and is installed. Thanks for all the wisdom shared.

MAKE SURE YOU ORDER THIS ONE!

http://www.autohausaz.com/search/produc ... al%20Mount

wkohler wrote:MAKE SURE YOU ORDER THIS ONE!

http://www.autohausaz.com/search/produc ... al%20Mount

I did.

Incidentally, it looks just like the dead one Guess this happened there before - wonder what is their lifetime.

Check your csb while your under there. Then look at your heat sheild ... Mine has a knock behind the fron seats and there's a huge area where the driveshaft is knocking the tunnel and sheild. Mine doesn't seem to do it in 2nd gear either...

If you have a bad CSB, you need a new driveshaft - or something is wrong.

One miracle at a time

The diff mount is in the mail, hopefully coming today, don't know when I can install it. 99% probability it is the culprit, from what I've seen.

I was having the same problem with my 535. Last weekend I replaced the diff mount and this problem went away. Too bad I ran across this thread after installing the $173 bavauto mount

California Dreamin' wrote:What car do you have?

If it's not the center support bearing (and I agree with above about accell vs. decel) then the next thing I'd look at is the harmonic balancer. If your car has one - not sure about eta's.

I think the HB (yes, many etas had em) would make scraping noises rather than thumping noises. Not to mention he mentioned middle-rear, not up toward the front (where the driveshaft would meet the output of the tranny).