Thump from rear suspension or engine

[Walter_Middie]

I recently had my engine/trans out, and now when I hit a bump, the car makes a noticeable thump in the back. My suspension is very worn, but it wasn’t doing this before I took the engine out. It sounds like something heavy is coming down on something solid. It is not a metal to metal sound, more of a thunk or heavy thud. I’d blame my suspension, but like I said, it didn’t do this before the engine/trans removal. It doesn’t happen unless I hit a big enough bump, or go over a speed bump. It doesn’t happen just driving around, or cornering. Here’s what I have done so far:

Checked the motor/trans mounts.
Hit everything in sight with a rubber mallet (could not replicate the sound).
Put a wrench on the upper shock mounts.
Removed the rear sway bar.
Tightened the motor/trans mounts.
Removed an exhaust bracket.
Removed and re-installed the motor mounts.
Checked the axles.
Jumped up and down on the back bumper (could not replicate the sound).
Tried to tighten the motor/trans mounts – did I mention that I checked the motor/trans mounts?

I’m out of ideas. Could I have bent/moved the rear mount bar enough to cause an interference between the engine and chassis?

[bpp944]

What year car? Sounds like a broken sway bar bracket to me. (but then again I hear that sound in my sleep)

[Flieger]

There are linear bearings in struts and dampers that can clunk like that.

[oneblueyedog]

Could also be the torsion bar spring plate bushings knocking, but mine made a clonking noise pushing down on the back bumper and going around a curve in reverse backing out of the driveway.

[IROC]

Originally Posted by Flieger

There are linear bearings in struts and dampers that can clunk like that.

+1. I chased a "clunking sound" in the rear for a long time until I removed one of my Koni shocks and it was dead. I could easily compress it by hand. I replaced the shock and the clunk went away. Might be something to check.

[Walter_Middie]

Thanks for the ideas - I'll remove the rear shocks and see if that gets rid of the noise. I've been wondering what is holding them together after all these years anyway.

[Fishcop]

The same noise turned out to be the rear wheel bearings on my car...

[Vincent 911]

I had recently the same issue. Same scenario: after an engine rebuilt I had a strange noise a the back right hand side wheel. It appeared it was an oil hose (from the engine to the external thermostat) touching the wheel.

Good luck

[sbrown.nw]

My 72 with a 3.2 does this on slow 90 degree turns. First thought it was a shock tower, then a torsion bar backing out. It also did it before I had either the front or rear sway bars mounted. Monty at Redmond European thinks it is a poly control arm bushing that is binding when it rotates and loads, and then shifts with a good "clunk or pop" to unload. Will be going to some different sealed almost monoball like bushings that allow rotation - about a $500 job with parts and labor...

Opinions?

Scott

[Walter_Middie]

Thanks to all for the good suggestions. I owe Mike Dayton and Flieger a beer. I've been riding my bicycle to work this week (despite the rainy weather up here), because the thumping was getting pretty concerning. I just removed the left rear shock, and my nice soft and smooth suspension is back. So - I need new shocks. Actually, I need all new suspension, but new shocks will get me back on the road. What should I get for a mostly stock mid-year that is only street driven?

[Flieger]

Glad to help but I'll have to pass on the beer. If you check out the "Mad Max" article in the August Excellence, you'll find out that I am too young.

What dampers do you have in there now? You should do left and right sides at the same time but can do 1 "axle" at a time. Most people reccommend Bilstein HDs in front and Bilstein Sports in the rear. I have heard that the valving on the Sport and HD fronts is the same in compression but that the HD is softer in rebound.

Gas charged dampers/struts will raise your ride height a couple inches if you do not currently have gas-charged. If you switch from Boge or Koni sturts in front to Bilstein, you must get new struts or modify your old ones.

Currently, I have Koni struts in front (original) and Bilstein dampers in the rear.

I reccommend going with the same brand and model of damper that you have right now because your dampers were specified from the factory to match your torsion bar spring rate. I would go stiffer than optimal if I had to make a compromise, especially if I were going to upgrade the suspension later.