This is getting more interesting (and confusing), but I think I may have diagnosed this as a muffler problem after all (although I swear on the first day that I heard the noise it was most definitely coming out from the RR side). Shortly before the cars two year hiatus in my driveway, I took it to get a sleeve welded onto the breaking pipe that leads to the muffler. I didn't get a chance to drive the car much since the axle broke not long after, but I now think the shop welded too long of a sleeve, that actually ended up pushing the muffler too far back, such that it's hitting the frame when it heats up and expands. I have evidence of this: the bracket on exhaust pipe end of the muffler (the one that has contact with the rear dampener) is protruding into the metal lip of the frame that comes down under the bumper, enough so that when I took off the muffler donuts (which were hard to get off, since the muffler hangers are pushed .5"-1" out of alignment from the hangers on the frame mount) the muffler was actually supported by nothing other than the frame lip!

So, at this point I'm fairly confident that this is what is knocking, or perhaps something else farther up the exhaust may have become loose as a result. Gonna do some more troubleshooting...I'll be glad if I can rule out the axles with more certainty.

gregszustak: I thought concrete blocks were known to crumble under the weight of cars?