Ok. I searched and only found one post about this problem and wasn't that helpful. No matter if the car is cold/hot idling/moving it makes this loud vibrating sound. I tried to hear where it is coming from and it sounds like it is coming right after the cat. convertor. If anyone has had this problem and knows a fix, please help it would be greatly appreciated. Thanks. (1993 Civic Si)

 

it's either an exhaust hanger that broke, or it's one of the bolts that hold your exhaust together is loose, or and this is what it probably is.....your cat.

Go under your car and give your cat a nice bang on the bottom, if you hear **** inside ratteling around then you need to get yourself a new converter.

 

heat shield

 

Its the heat shield, i had the same problem before, and there are actually three of them, one is right after the cat, the other two is on the rear right wheel. try to remove all of them. its annoying especially when you back up the car.

 

its the heat shild