Well, I'm not an acoustics engineer or any other kind of engineer, but it sounds like you have a passive radiator problem. In other words the panel or componenet that is vibrating is not being excited directly, but is just transmitting the vibrations.
Used to be a stereo speaker back in the 60s or 70s that use this idea to "enhance" bass frequencies.

What you have to do is add mass (undercoating, stick-on insulator, etc to the panel or item, or support it more ridgidly. Something to get the culprit's resonant frequency away from the exhaust system frequecny

The panel or componenet might be behind the headliner or unedr the carpets or in the trunk. Problem is to find it.

Or do what I did after my second set of glass-packs: ditch the duals and glasspacks and install stock single exhaust. Ahhhh peace and quiet....

I'd go for undercoating for sure.

My $0.02