ORIGINAL: Mr Cox
Could also be the clunk bouncing about and getting air into the fuel line when the level is lower, so check for bubbles in the line. The tank should only touch soft foam and not the airframe it self, you'll also need a well balanced prop to minimize vibrations.
I have heard this preached so much, but have never seen it happen in my planes. Both my trainer, stick, and extra's tanks are wedged in tight against the airframe, and they will all fly to a dry tank. I have to believe that either this was just a problem with older fuels or is basically a myth.