Steele-Bike, I was biking with a friend of mine this weekend and we experienced the same creaking you explained.

Turns out, the problem was in the connection of the crank arm to the axle.

It seemed tight, but it still creaked. I isolated the trouble by grabbing both crank arms and simultaneously pulling them both up and pushing them down. The exact crank was identified by pedaling only one crank at a time.

I removed the crank arm, cleaned off the crank arm, axle, and cotter (ya, it was an older bike), put a touch of grease on the cotter, and counter-sunk it in with a punch and hammer.

That worked.

Did you ever find the cause of your troubles?