no my cats arent burned out, i just had them replaced, but whenever i go through a puddle and accelerate or downshift and slow form 4000RPM i can hear the what sounds like the downpipe and cats shaking and making vibrating noise. these were not OEM cats. they were magnaflow universal car and i had the guys (good guys, dealt with them before) put on new flexpipes. the only difference i can see in the setup is that the down pipe and cats are one piece stock and the new fitment has the flexpipes maybe half an inch shorter than stock. the exhaust isnt any louder, it just rattle/vibrates when the system get wet or im engine breaking from a high rpm. and guess as to whats wrong

 

I had just experience this just this morning on my way to work. It was raining and I was on my way to work and was driving through some puddles in the Right lanewhen I experience the same low vibration. I moved to the left lane and coasted a little and then accelerated and everything was fine again. The vibration/sound to me came from the down pipe/flex pipe. I have a APR exhaust, but don't know what had happen as to why it vibrated in that assumed area of the exhaust because of contact with the water. I was wondering when I got to work if some water was getting suck in through the mesh. Is this possible? Awhile back prior to my timing belt breaking[:@], I used to have that problem your talking about when coming of the gas at high rpm (same exhaust). I found out that I had a evap code??(got me)The mechanic found the issue with the code, clear the code; scanned again and no evap code. The car when I got it back and now has not vibrated at high speeds/rpms. Hope my experience helps. https://www.audiforums.com/m_503081/tm.htm

 

Maybe a few pieces of the cat got farther down the line?