Unusual Clunk on Pedal Start

[purpurite]

Recently picked up a winter bike that is a full Shimano 105 setup, and I have overhauled the whole bike to near perfect condition. I threaded in pedals to take a test ride this morning, and was surprised to hear a strange heavy, hollow thunk coming from the bottom of the bike, on initial pedal force. It doesn't happen every time, but every so often on pedal start. It's not a sound I have ever heard on a bike before.

I pulled the cassette off of the rear hub and cleaned everything and reassembled, and tightened the cassette properly, and the sound still exists. My next step is to disassemble the bottom bracket and see if there is anything unusual there. It's a TruVativ GXP BB, and I know they aren't favorites of many, so it might be something failing there.

When I got the bike, it was visually not maintained, covered top to bottom in mud, grime and a little rust on a few steel parts. I know that there are things that I will uncover as I go, but this one is confounding me. I can't tell exactly where the sound is radiating from, so I'm throwing darts with my eyes closed trying to guess what the sound could be.

Anyone have a similar experience or have any ideas? No other sounds or red flags anywhere except the front of the first pedal stroke.


Guesses?