Tank venting / periodic hissing noise

[munition]

I hope that someone here is maybe able to help.

After 20-30 minutes of driving, I hear a regular hiss of about 2 seconds duration from the rear / from the trunk. The hissing goes along with the opening of the fuel tank vent valve, so whenever the fuel vapor is fed into the intake. I suspect there is too much vacuum in the tank.

I thought at first of a leak in the lines or at some connectors and therefore I used my smoker / leak detector to inspect the vent lines the tank. Everything is tight, not a little sign of leakage.

My questions: Is there always negative pressure in the tank while driving ? Is there a breather valve somewhere to let air into the tank or is there a kind of safety valve if the negative pressure gets too strong? Or do I have overpressure and it pushes the fuel mist out somewhere (I don’t recognize unusual fuel smell)?

Any help is very appreciated.
Andreas

[Alfan]

Is this happening when you turn off the car? Is it causing problems?

I don't recall this happening for any of my cars, but when opening the fuel tank cover, it can be very different, if there is audible pressure regulation or not...