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Thumping from under hood. Torque strut mount or Engine mount or Trans mount?

Wondering if anyone would like to help me tackle this question.

I have a well taken care of '95 Buick LeSabre w about 80,000. Bought it a couple years ago from a man who kept it in his garage. Most components on car are original.

Recently I was driving down a very steep driveway at about 3 MPH, and bottomed the car out and scraped the bottom. Not too bad, but enough to make me cringe. As I drove home at about 45 MPH, I noticed that every time the car would engage the gear, there would be a thump coming from under the hood. This was at about 1300 RPM. After looking under the hood and having a buddy rev the engine while in drive with the brakes pressed down, the engine doesn’t seem to move too much. Furthermore, the engine mount and Trans mount seem to be okay, and not broken. Rubber is solid.

Is the thumping sound perhaps due to ‘old’ mounts or could this be something else like the torque strut mount,etc? Also, will it be evident when one of these are damaged, or is it something you replace after a certain while like shocks, etc?

Check for play in the mounts in reverse. In fact, have your friend rev the engine to 1300 rpm and shift repeatedly between reverse and drive while keeping the brake held FIRMLY.

I’ll try that out. Here is a vid I made yesterday of the engine. The second time I tried to test the play later that day, I didn’t get half the movement I got on this original video…even when I pushed the pedal down more. I tried it a second time so that I could get an RPM reference, which I had forgotten to pay attention to the first time when I filmed it.

As requested. Engine engaged in both Drive, and Reverse. RPMs didn’t go past 2000. Did seem to move quite a bit. Perhaps the mounts are getting bad? But if so, how do we explain why now it decided to thump under the hood, and not a week ago?

Thanks.

You said it thumped when you changed gears? Any chance you knocked a hole in the transmission pan and are losing fluid? I think I might want to get it up on a lift to have a look at it.

Update

I drove the car to work today (just a couple miles going about 40). It didn’t make the thumping sound at all on the drive there. On the way back, it was getting warm, so I put on the AC. That is when the thumping started. My buddy and I tried to troubleshoot it while we drove. It only makes the thumping sound when the AC is on. It ceases completely when the AC is off. The thumping is not terribly loud, and happens every 10-15 seconds. It feels as if we went over a little cusp on the road; just enough to give the car a quick and minor jolt.

This is good, because we are now 99% certain that this problem no longer has anything to do with any mount whatsoever. This is an HVAC issue…at least I think!

The car does not thump when changing gears. As long as the AC is off, she runs fine. We alternated for a bit turning the AC on and off…perhaps 1 minute intervals. As soon as the AC turned on, it would resume. I might have initially been led to think that the thumping came from the gears engaging due to the fact that when I first heard the noise, I was driving on a long straight road for miles, and the noise was consistent in its intervals.

Check the harmonic balancer.

Should I be getting a consistent noise if it were the harmonic balancer? It just seems that the problem with those seems to be an ugly wobble and mine seems to be alright.

When the harmonic balancer fails it is usually noisiest at idle and when tipping the throttle off idle. There may be no noticeable noise when driving at a constant speed. A loose balancer can rattle for many miles before a wobble occurs and as soon as it wobbles it will toss the belt.