It sounds like fuel check valve is loosing its function.
Check out this item (it should vibrate when you hear this sound):
I have same issue from about two years, but it doesn't do anything else other then "farting" from time to time (especially a minute after turning off engine) so I don't bother.
People say that if this valve is broken, it may cause long cranking times when starting (fuel line looses pressure), but in my case it is not happening (starts normally). I think it is on evaporation tube - so it does not have to do with fuel line pressure, but it releases pressure from fuel tank.
Regards,
TD