Hi JMLee

I had a similar problem about a year ago, which turned out to be severe pitting of the cones in the rear wheel bearing. One would expect this to have made noises all the time, but it took the extra forces due to pedaling to provoke it. This is what confused me, before I found and fixed the problem I'd checked the cranks, chainwheels, had the bottom bracket changed, looked at the saddle and seatpost...

Just a thought,

Ed