Wheel skewers!

I was so sure that my bike's creaking was in the bottom bracket. It only happened when pushing fairly hard on the pedals. I finally found that the rear quick release was making the noise.

I could make it creak somewhat by standing next to the bike and using my foot to push down and inward on the pedal, flexing the bike frame. It seemed that the noise was toward the back of the bike, but it's really hard to localize creaking noises!

I disassembled the skewer, wiped it down, put a light film of bike grease on the cam, and used a little more force when tightening it. No more noise.

Another rider mentioned the same problem on a recent ride.

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How much more tension on the spokes? That doesn't sound like a good idea. I think too much tension can lead to cracks around the spoke holes.