exhaust noise - rattle rattle

[salansky]

After replacing my water pump *again* due to a leaky seal and driving my 540 just a week, I now have an annoying exhaust rattle. It started only at start up and only when cold, but now it rattles upon acceleration and whether hot or cold. The sound is coming from directly below the driver, probably at the cat(s?) and sounds like loose marbles.
What do you experts think is the verdict here - did the cat finally give up? Are there 2 on my 540, or just one anyway? I never really took notice and have not had it up on a lift lately.
Any idea what this is going to cost?
Sorry for the bad attitude, but I'm kinda pi55ed about the timing.
TIA,
-jack

[Kalevera]

Hi Jack...

First things first - I'd get under it and visually inspect it. My exhaust rattled when the rubber doughnuts that support it wore out; in fact, one degraded so much that it just fell off. Same exact symptom from the same location - rattles on startup, then it's fine once the exhaust heats up. Assuming your car has the same type of setup with hangers for the exhaust, it may just need new ones. They're about $2/piece from BMA and my 535 uses two around the exit of the cat/resonator and two support the muffler. Yet another device is used to support the exhaust from the auto trans, which could wear out and probably cause the same sound.

EDIT: oh yeah, and when you get under there to look at the hangers, shake the cats and see if they rattle.

best, whit

[Alexlind123]

You might want to make sure the heat shields are properly secured.

[genphreak]

yep I second that, heat shields are key- the cats move when the engine is running and under load. However visually inspect the cat covers, if they have been hit from below for example- they is the most common cause, esp. in lowered vehicles. once they are loose- esp. the welds at the input and output flanges they make this noise. If so jam a little something in there and check for a reduction in rattle. Very hard to weld if so as it is 0.6 mm sheet alloy-steel, it just goes again... ur only chance is to reweld in the same places as stock (2 spots at the front, 2 spots at the rear of each cat pipe) as the expansion rate will tear welds made elsewhere and wreck the job on the first run...
:) GP

[Nzimiro]

You might want to make sure the heat shields are properly secured.
I had similar problems for the past few months. Went under the car over the last weekend and found loose aluminum heat shields (located between the exhaust manifolds and the CATs). My car was also shaking badly at idle - turned out to be broken exhaust support clips. Replaced them and plugged all leaks detected on the exhaust system - car has gone quiet again, with more horse power.