i had one bike that creaked and we could never find the problem, but it turned out to be the cartridge bearing in the rear hub creaking (outside of the bearing where it's pressed into the hub).
also could be the actual QR where it clamps to the dropout.
or a zillion other things.
my SS road bike has a creak I have yet to track down. nice thing about SS is there are far fewer parts that can creak. OTOH, you pedal a lot harder so it creaks more.