That's happenning to me too. It got stronger when I lifted the cap... If I could only find some matches, I'd take a look inside - but I think I'm gettin' closer to the source.
No, really- (just ribbin' you) Sounds like you might have a leaky hose connection. Maybe the hose is not all the way on straight, or the clamp is crooked. There're probably several- the overflow line and/or vent line, and the main pressurized fuel line. If you did any work recently this would be a good place to start. You can probably see it.