I got my fixed gear out of storage, where it's been for four years. Everything seems fine, except when I ride with anything more than light force on the pedals, I get a kind of br-r-r-r-r-r noise. It definitely corresponds to the degree of force I'm pedaling with (louder during the part of the stroke where I'm pushing hardest).

I vaguely remember running into this before, and the problem was having the chain too tight. But I've set it to have enough slack to be able to lift it up about an inch. The chain is lubed and in good shape. It really doesn't sound like bb noise.

Any other ideas? Thanks.