Loud Vibrating/Exhaust Noise @ 2300 RPM

[timw300]

Just bought a 2003 Rondezvous (3.4 V6 CXL AWD 60,000 mi) runs well except for a very loud muffler-type noise when you reach 22-2300 RPM, and only at that time. Before and after these RMPs are fine. Dealter just had brand new factory muffler installed (original muffler went bad) but this noise is still there. Took it back to check closer, and all parts seem to be in good working order. Tested many things on exhaust but problem persists. Amazingly noise is louder on inside than outside vehicle. It is not general road noise as far as I can tell. Any thoughts???