My wild guess...
Look under the floor, on the bottomside, and be very sure that all of the rubber plugs are firmly seated in the undebody.. Or maybe with the carpet out, you can look from the top..
If you don't know what I'm talking about, I'll try to explain it better..
89 300SEL Money Pit
92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week
88 Wagoneer Slightly modified (Not for soccer moms)
04 Kia Sedona with every option... NICE