There is a loud rumbling under my car that I am very worried about, but it comes in and out at random times, but it also only happens when its raining out. It has also only happened during the last few days it has heavily rained here in NJ. Im pretty worried about the sound, anyone have an idea as to what it could be?

 

downpipe rubbing cross bars or hitting the sway bar and it may be the exhaust hitting drive shaft. those are the 3 i can think of

 

My driveshaft was hitting the resonator on my exhaust - and i heard it mostly when raining.

 

Rear u-joint on exhaust.

 

https://www.evolutionm.net/forums/ev...ring-rain.html

 

i'd get under there and start tightening bolts first then go for another run to minimize location =]

 

But why would this be happening just when its raining

 

Your exhaust is say 800 degrees or so depending on how you're driving. Now splash cold water on just one side, the temp drops a few hundred degrees on that one side and it shrinks causing the pipe to bend toward the cold side.

Coefficients of Thermal Expansion for stainless steel is pretty high.