Most modern E.P.A. Compliant vehichles are euipped with an anti evaporative fuel system, which is meant to keep the fuel vapors out of the atmosphere, these systems reuire a gas cap that has a total seal up to 14 psi. The hissing you are hearing is not pressure, but rather vacuum;air sucking back into the tank, to compensate for the physical volume of fuel which has been consumed, and the crunkle sound is often the large flat portion of the tank(the part most sensitive to changes in atmospheric pressure)returning to normal, almost all modern cars do this to some extent.
Tuesday, March 13th, 2007 AT 3:30 AM