VoigtClub Messages: 15 Registered: October 2012 Location: Cambridge, MA
Chancellor
Yup, resonance. At those lowest "felt not heard" frequencies the pressure in the vehicle is yanking and popping the thin metal walls of the car 18-25 times a second.
The main product line for combatting panel resonance is called Dynamat, which is a tar-based tile that adhere's to the inner car body and dampens it through both mass and viscous resistance.
If only every DIY installer understood that your car isn't supposted to fart and installed some dynamat, I'd be much happier about those bass-head idiots cruising by.