I get this A LOT on my bike that has a quill stem with adjustable head angle. It has 2 screws on the clamp around the bars and anotherone holding the adjusted angle in place. I've tightened everything up to max torque and its still there.
I think its a mixture of metals thing.
I've always had this problem on this particular bike. change of stem and change of bars has changed it to a greater or lesser degree, but its never cured it.
Its definitely the bars/stem clamp. Very annoying, but I can't cure it.
Stew
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The older I get the better I used to be.