Thanks for all the feedback fellas!

I briefly checked during lunch and I don't see any obvious cracks in the chainstay or BB areas, so I'll have to apply some stress like DiabloScott & 79pmooney said and check again just to be sure. I'll start taking some of it apart to see if it's a sleeve, hub, or axle. I wish I had a spare rear wheel so that I could simply swap that on to see if it eliminates things (which would help me isolate the issue). oh well. lol

If I don't see anything obvious, I'll likely just drop it off at the LBS tomorrow and then let you all know what it was. Either way, more to follow...