hmm,

looking at the rpm your quoting, and playing with some maths if what your hearing is a 'resonant frequency (i.e. a standing wave) your looking at something with a wave length of 11.09cm long...or something with a harmonic on that...

so take 11.09 and look for something that long or a multiple of that (probably an even multiple - but not neccesarily)..

usual disclaimers apply - plus school room physics was a long time ago..

My '91 740 made a very annoying "booming" resonance drumming exhaust type noise, just in the 3000-3300 rpm range.

New hydraulic motor mounts solved it for me. The engine pan was just starting to touch the rubber bumper on the crossmember.

As I recall, the clearance 'tween pan and x-member had shrunk to about 1/2"-3/4" -- although it was a few years back....

--
Don Foster (near Cape Cod, MA)

I'm not sure about the particular replacement system you purchased, but it's been my observation that non-oem replacement exhausts for Volvos are consistently disappointing. It's possible that what you bought is simply underengineered or not engineered at all, and thus is subject to resonance.

Have you checked your motor mounts....could be they're just at the end of life.
Also, IIRC, isn't there supposed to be a bracked around the downpipe at the bellhousing....if it's rusted off or broken that could contribute.
Perhaps you put the driveshafts together slightly out of sync...they have to go back the same way (the two driveshafts behave like a constant velocity joint...the angular accelerations peculiar to one pair of Hookes' joints is cancelled out by the second pair so the rear end sees a constant angular velocity) or they vibrate.

Otherwise maybe it's the old 'non-volvo part' syndrome...if it didn't resonate before you put in the system and now it does, then it's the system itself.

Perhaps put it up on jackstands, run it up to resonant frequency in drive, and see what is drumming. There are ways to alter or suppress the resonant frequency, depending on what is drumming. If an exhaust component is touching the body you may have to bend something. All the exhaust components should be hung by rubber doughnuts, except of course the connection to the exhaust manifold and the aforesaid bracket (if fitted).

It sounds the like the phenomenon of exhaust resonance. Some poorly designed new cars have this. It's just a coincidence of factors that results in a strong reinforcement of certain frequencies at certain rpms. I wish I could be more helpful but I don't know any quick fixes. I would guess that adding an additional muffler, maybe a low-restriction straight-thru design, might eliminate it. Have you consulted a muffler/exhaust expert?
--
'84 "285" rice-eater with lotsa hp & performance goodies