Finding an exhaust rattle

[JHZR2]

My auction win MB 1991 350SD is being baselined for all fluids and whatnot so I can use it regularly. Had it out today. Very smooth, good power. Love the 3.5L OM603 in a w126 (the w140 is too heavy for it).

It cruised along great, but I find that right around 2000 rpm, I start to get an annoying rattle or vibration, that comes from the left rear. This is where the exhaust routes.

We noticed it a few weeks ago, my 10yo recorded their finger while taking the video. Not that there’s much to see. Today they did a little better. You can hear it pretty readily.





It’s not related to the driveline. If I take away throttle, the noise goes away completely. This has been absolutely proven. I also verified that I can make it happen when parked, just taking RPMs up to that level. Thing is, inside the car we can hear it. Outside it isn’t detectable.

I’ve shaken and manipulated the exhaust system all sorts of ways. Nothing has caused such a noise. No metal to metal or anything else. Just a bit of squeaking at the far ends of movement.

So. It sure how to diagnose and find. Any recommendations? Perhaps it isn’t exhaust at all, just some other resonance or moving part?

What’s the best way to diagnose something like this?

Thanks!

 

[Kira]

See if none of the 32 year old engine mounts are collapsed.

 

[andyd]

Total wild guess Is the inner wheel well liner / !/4 panel adrift

 

[PontiacHO]

Baffle loose In muffler.

 

[JHZR2]

See if none of the 32 year old engine mounts are collapsed.

Definitely a thought. And on my to do list. But I’d think that if one was collapsed I could cause a rattle just shaking the exhaust. Maybe the torque shifts the engine further.

 

[JHZR2]

Baffle loose In muffler.

Could be, but wouldn’t I be able to rattle it then in another condition? Even if I hit it with a rubber mallet?

 

[PontiacHO]

Don’t rule out anything. Heat shields, hangers, and rocks etc collected on top of shields.

 

[Aredeeem]

Heat shield. Pin it back into place or better yet remove it. Have run without one for six years. Don’t park on leaf piles. Next!

 

[Soft Cars]

See if you can hear it going through a drive-thru. The sound bounces off the wall from the building and amplifies the sound.

If so, then it's probably a catalytic converter with its guts rattling inside. True story.

 

[Warstud]

It cruised along great, but I find that right around 2000 rpm, I start to get an annoying rattle or vibration, that comes from the left rear. This is where the exhaust routes.

Have you checked it while revving it in park around 2000 rpm ?

 

[Dave9]

So what if you just throw another exhaust strap on it and see if that fixes it?

Doing that doesn't need another hanger welded on, just grab a roll of steel strapping for the pipe interface, decide if you need another piece for the body connection, then a rubber doughnut isolator bolted between the two... or just replace the existing rubber isolators if they are shot.

 

[JHZR2]

Have you checked it while revving it in park around 2000 rpm ?

Yes, it happens then exactly the same. I can hear it inside the car, but can’t hear it outside.

See if you can hear it going through a drive-thru. The sound bounces off the wall from the building and amplifies the sound.

If so, then it's probably a catalytic converter with its guts rattling inside. True story.

This is a diesel, no catalytic converters.

So what if you just throw another exhaust strap on it and see if that fixes it?

Doing that doesn't need another hanger welded on, just grab a roll of steel strapping for the pipe interface, decide if you need another piece for the body connection, then a rubber doughnut isolator bolted between the two... or just replace the existing rubber isolators if they are shot.

Exhaust strap on what? I can’t pinpoint the sound, that’s the fundamental issue.

 

[Dave9]

^ Exhaust or exhaust shield. What else could it be if coming from the left rear even when parked? Maybe muffler if that is its location, or 2nd mini-muffler aka resonator if it has one.

How about having a helper rev the engine while you place a rigid object on the muffler to see if you can feel it? Another old timer's trick is hold a length of hose to your ear and probe around with the other end, though most hoses aren't especially temperature resistant.

 

[D60]

I'd probably lock the throttle at 2k and start climbing around the back seat, under the car, whatever (I understand you can't hear it outside, but still). Touch everything you can to see if you can affect the noise. Wear welding gloves if you need to touch the exhaust. May need a helper sitting inside while you're underneath. Perhaps doing this next to a building or wall will help if it does indeed reflect the noise.

 

[JHZR2]

I got under the car. This one mount was noisy. Worth a shot…

This one can be swapped too. Though it isn’t noisy.

 

[Snagglefoot]

You might run your hand along the top side of the muffler to check for holes.

 

[JHZR2]

You might run your hand along the top side of the muffler to check for holes.

There’s some corrosion on the muffler itself. Which is surprising since the car is very clean and rust free. But nothing on top that I could see.

 

[Sedin26]

I recently had an exhaust rattle that i just couldn't find. It only happened on heavier throttle, moving or parked, and I couldn't find anything that was rusted, had holes, or was loose. All joints looked good.

Eventually, I got under again and had a helper rev it up - I found that at what i thought was a tight joint, it would actually open slightly under throttle and would rattle a heat shield nearby. I couldn't even see it initially but noticed water dripped out under those circumstances.

These can be hard to find sometimes and don't always end up coming from where they "sound' like they are

 

[PimTac]

Maybe it’s not the exhaust system? I would check both underneath and inside as well. The problem might be something you never expected.

 

[JHZR2]

Maybe it’s not the exhaust system? I would check both underneath and inside as well. The problem might be something you never expected.

Very true but knocking on anything and everything has been no help.

And it isn’t movement dependent which is a if help to reduce the options.