AJU

I have spent a lot of time reading all the BB noise threads I can find (not so easy without the search function) and haven't quite found the answer. I rode my bike, Schwinn Mesa GS (I think it's a '99), in the rain to work the other day. It's got fenders, but no mudflaps. On my ride home, I noticed that there was a click/groan (can sound like a series of clicks on each side) every time either pedal was in the forwardmost position. Here's the strange thing: it isn't there when I'm just spinning to keep up with the wheels, but it goes away when applying a large load such as when I'm really cranking to get up a hill. It's really disturbing because the sound wasn't there before, and after the rain and only a couple hours time it was. Doesn't seem to me to be enough time for rust or any corrosive damage to occur.

What I've done so far:
Tightened the crank retaining bolts
Tightened the chain ring screws
Tightened the seatpost
Greased the chain (with WD-40, I know it's not the best, but I wiped it down well)
Sprayed the BB area with WD-40
Tightened the skewer on the rear wheel
Checked for vibration between the fender and the frame

BTW, there's no side to side play in the BB or crank that I can detect with my hands.

Any ideas?