O great wisdom of Volvo wizards, I need your help.

I've got a 91' 245 with 175k on it and some possibly expensive repair may be in my future.

while driving a day or two ago a new noise happened while I was accelerating at around 10-15mph. it was a brief light grinding noise. No vibration in the gas peddle or the car, it happened even more when going up a hill at slow speed. but, the grinding only happens at a specific lower speed and doesn't occur in between gears. After driving a while it doesn't seem to occur, or is not as noticeable. Then again, my muffler is starting to go bye-bye and it's hard to catch in noisy traffic. my best guess is either the wheel bearings or even the rear differential, or a long stretch could be a reverberation of the muffler at a certain point (but that i think would occur on all of the gears when the muffler reaches that frequency, unless of course the muffler doesn't hit that frequency in the other gears and it wouldn't be make a grinding noise would it?).