Check & tighten the pedals. Does it happen on one downstroke, or both? Tighten the crank bolts. Tighten the rear wheel skewer. Remove and reseat the bottom bracket - I find plumber's (teflon) tape to be a good solution (as FBinNY posted).

My g/f's DB sorrento had a "semi-cartridge" bottom bracket. One side was a cartridge, the other was loose bearings. There was also a distinct clicking when pedaling hard - almost like a ping, felt like your shoelace was hitting the frame or something. Anyway, tightening up the loose bearing side with a spanner made everything go away. If it's a cartridge I'd say you're SOL and have to replace it though, if reseating doesn't fix it and there is play in the BB axle.