Why is my tank popping?

[MikeL]

On four occasions on the last couple of days while riding my CB-1 I've heard a loud popping noise from the fuel tank, the sort of noise that sheet metal makes when there is a sudden thermal expansion or pressure build-up. Funny thing is, I've had this bike for four years and it's never done it before. Externally the fuel tank looks fine, the bike behaves perfectly normally, no fuel problems, absolutely nothing else wrong. When it first happened the tank would have been about 3/4 full, now it's got about 1/4. Opening the filler reveals nothing.
Am I right in assuming that it is due to a pressure differential? Could it be caused by a blocked vent? Or what?
Any suggestions?

[frogfeaturesFZR]

Probably a blocked vent, but then the motor should evevntually get starved of fuel and stop. Dunno..... is there a vaccuum in the tank, ie does air rush into the tank when you open the gas cap ?

[j_redley]

Also double check your breather pipe, make sure its all clean. Could be either kinked....