I know some systems pressurize the fuel tank. My 68 has a fairly new stock gas cap and it was "moaning" after driving for 15 minutes. Took me a min to figure out what the noise was. Turned gas cap(carefully) and whoosh! seemed to be an inordinate amount of pressure. I could also smell gas while I was driving. First thing I did when I stopped was look underneath for a leak. none. is the cap supposed to be vented or not? the tank has both the fuel line and vent line attached to the fuel pump.