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Noise in the rain

My '02 Impala makes a low moaning sound only in heavy rain and only if I am forced to drive through a large puddle.
It first occurred a year ago after it had it’s catalytic converter replaced due to the recall. The dealership mechanics checked their work and could find nothing. I tried spraying it with a garden hose under the car and under the hood while moving the throttle, still no sound. A local, and very thorough mechanic, heard the sound on a rainy day, but could find nothing. He could see nothing wrong, but thought that changing the serpentine belt would help. I replaced it with one from the dealer, but the sound still returns on rare occasions.

This noise is common on a lot of different cars. You mentioned rain and puddles. The noise is caused by the contraction of hot exhaust components when suddenly sprayed with water. The component may be the muffler itself or possibly a heat shield. There is no simple cure, but the effect is completely harmless and of no concern.

I believe Steve has explained it. You could try having the exhaust system examined to ensure that it’s secure and free to move about, but it’s a crap shoot.

This exact problem was one of the litany of problems that I had on my Volvo.
The exhaust manifold had a slight crack in it, and normally there was no noise emanating from it.

But…if you drove in the rain, at highway speeds, it would raise an awful racket.
Yes, I know that I should have replaced that manifold, but when everything on the car is deteriorating before your very eyes, you have to be somewhat selective regarding what to replace…and when to replace it.