Engine noise "transmitted" to body - why?

[E_S]

Around some certain rpms, a real resonance and vibration occurs in my car. (W124, the one in my signature) I was thinking the engine mounts were shot after 16 years and replaced them. It made a difference, but did not change anything about the noise level. I've double-checked that the exhaust does not touch the body and the heat shields does not touch the exhaust. The noise is not exhaust noise, it's clearly a resonance and noise from the engine.

I can't figure this out other than the rear transmission mount. Any opinions? Right now, the car is as noisy inside while doing highway driving as an 70s econobox..