vibration in the rain only ?

[mikehonda]

why does my 91 240 manual have a horrible vibration on the highway in the rain only ? it comes from under the seats almost . sounds like hell ,or if something is going to fly off !

[180fan]

sounds more like the heat shielding or something if it's from just below you

[RB280SX]

I believe that since your car is very hot after driving on the highway that the manifold is shrinking in size (I thought manifolds and such expanded by a tech told me otherwise. Although it doesn't make much sense, he did what I am going to tell you and the problem went away), causing the downpipe/testpipe to get right next to the undercarriage. The solution to this is to jack up the car and MAKE SURE THAT IT IS FULLY HELD BY JACKSTANDS, and get a crow bar and bend the piping down. Another alternative is to get different hangers (ones that will let the exhaust sit lower) for the exhaust.

If I were you, I would first wait for the odd occurance to happen again and then jack up the car to see if the testpipe/downpipe is actually vibrating on something.