My 02 DTS has an intermittent loud buzz or whine from right rear speaker. After a dealer failed to figure it out (including changing the speaker), I found the GM TSB summarized below. It calls for changing the factory amp under the back-seat, but since the car is way out of warranty, GM won't pay so I live with the occasional sound. Can stop it by shutting off the radio & it usually does not come back with next start up. I know your car is a 95 & not covered by this TSB, but I thought it could be similar. Also mine is unrelated to speed.
CADILLAC / DEVILLE
Service Bulletin Number:
NHTSA Item Number:
SOME CUSTOMERS WITH VEHICLES EQUIPPED WITH THE NAVIGATION SYSTEM MAY COMMENT ON AN INTERMITTENT LOUD HUM / BUZZ NOISE COMING FROM THE REAR DOOR SPEAKERS. *TT