I noticed a distinct "thunk thunk" coming from my bottom bracket area (bottom knocking? ), looked down when it was happening, and noticed the chainrings moving in and out maybe 1-2 mm (at most) in time with my pedal strokes.

Got home, turned the bike over, and the crankset does move a bit when torqued side-to-side. The bottom bracket cups are solid in the frame - they're not moving at all.

The bottom bracket and crankset are Ultegra 6700, probably 10 or 12 thousand miles on the bottom bracket, less than half that on the crankset. (I could figure it more closely, but that's probably close enough)

Time for a new bottom bracket?