Hi guys...

I have a weird knocking noise like pum-pum-pum coming from the rear and only when accelerating (over 4k rpms) and going thru uneven surfaces.

It's only while I'm pushing hard and it's driving me nuts.

What you guys think this could be?

Transmission mount.

do a good visual inspection of all mounts trans, eng, diff and the suspension hardware. Start easy after that and change your differential fluid. Look at drained fluid and the plug for any metal chunks.

Any oil leaks from rear shock absorbers ?

Exhaust mounts ?

No oil leaks from the rear shocks... its very weird, only when the car is over 4k rpm the noise is heard.

Exhaust mounts inspected.. All good.

Differential mounts???? Where are they?

Replaced 10k miles ago

Check CV joints on the rear axle for grease leaks & seizure. Check Flex discs, UJ & centre hanger bearing on the propshaft.

Check all rubbers on the rear subframe & diff droop angle at the pinion flange.

Hi Glyn

I checked every single rubber on the rear axle, everything looks just perfect, no leaks or anything in the diff or the cv joints. Flex discs - great shape (front and rear), transmission mount - perfect, exhaust mounts - perfect.

Glyn, what exactly is "centre hanger bearing on the propshaft" and "diff droop angle at the pinion flange"?

If the nose of the diff hangs down too low it can cause the propshaft to whip under high torque. If all your rubbers are OK then it's not that.



Center hanger bearing is 100 in the diagram. Look for cracks in the rubber & excessive play.

Are the tyres & rims standard ? Any rubmarks under the wheel arch?

No, the rims are not standard...

I have 8.5 front and 9.5 rear with 245/40 R18 up front and 265/35 R18 rears... There is a very tiny rubber mark on the wheel arch right next to the bumper.

How are you running 245 on the front? What offset? or are you running spacers? 245 should touch the front strut even at ET32.

I'm using a 18x8.5" ET30. No rubbing at all in the front.

OK - ET 30 makes sense. The clearance at the vertical strut dimple must be damn close. On a 17" with the higher sidewall it would still probably touch because the sidewall bulges more.

You pics don't show clearance between the inner sidewall & the vertical strut.

I mean here - not a Merc pic.

I'm sorry for the delay.. I've got the pics here.

About the noise, I'm almost sure it is the center hanger bearing, because now I hear the noise more under my butt that in the rear. I already bought the part and waiting to have it installed.

Glynn, a curious question.. My car is rubbing a bit in the rear right against the fender clip and the liner. I have 18X9.5 ET35 with 265 / 35 R18.

I have enough space to make the wheel come inside the car a bit more so I can release myself of the rubbing, but How can I do that?

I'm surprised at the clearance you have to the front strut. I wonder whether those wheels are really ET30 & not something closer to ET 27. That said - the track was widened marginally on 2005> facelift cars & that might account for it.

The only way to bring the rear wheels in if you are not running spacers that you can remove is to machine the clamping face of the wheel.

If I machine the clamping face of the wheels, I won't have any alignment issues after that?

Replaced the driveshaft support and bearing, no results. Going to replace next week the engine and tranny mounts.

I have the same problem in the rear. While in motion, Do u hear like a slight rattle each time u hit a bump on the road.?

Yes, like that but under acceleration only.

Just to update the case...

I replaced the driveshaft support and bearing, replaced struts and nothing happened.

A few days ago I started feeling a very weak vibration at the wheel and seats, ordered engine mounts because I have that vibration and installed them yesterday.. Noise disappeared.

I don't know why me and everybody on the car felt the noise coming from the back of the car. But yes, it was just engine mounts broken.

Thanks everyone for your replies.

I'm pleased it's fixed. Cars do funny things when it comes to noise transmission. Anyway - you should be good to go for a long while now!

I was planning to do engine mounts and flex discs during the first quarter of next year. I still don't see how the noise my car makes under acceleration can be from engine mounts, though.. seems clearly from the middle rear of the car. I guess we'll see.