My back wheel is giving out a periodic squeak as if the wheel were buckled and rubbing on the brake. Only, it's not buckled and it's not rubbing on the brake.
It's one squeak per revolution of the wheel, regardless of speed, but each squeak is the same (short) duration even if I move slowly. It doesn't matter whether I pedal or coast. The wheel isn't rubbing on the frame and if I gently brake through several revolutions it still squeaks while I'm braking.
I asked a bike shop about it a while back and they couldn't find anything but after all the routine adjustments it went away for a while. Now it's back.
If I get off and turn the wheel by hand there's no sound. Even if I lean on it and roll it around it's quiet. It seems that I need to put all my weight (an my panniers and gym gear) on it to make it squeak. I guess that's why they couldn't find it in the shop.
My best guess is uneven spoke tension might allow some of the spokes to shift and rub slightly. I've looked them over and don't see any wear and I'm not sure how I'd make a definitive diagnosis there.
What could it be? What else can I test?