Completely normal. It means your gas tank vent is working like it should. The vent valve holds 50 Bars+ -. Over 50bars it open and vents into the charcoal canister.

My Euro 500SEC would not pass CA Smog Evaporative Functional Test because the tank would not hold 35 bars. It was the tank vent line hose to the valve that had a hole that could not be found until on a lift.

BTW that valve is found somewhere behind the right rear wheel in the well.

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1986 Euro 500SEC "RUF" 9:1CR, "Rose"
1985 Euro 500SEC Cabriolet AMG Widebody
1982 Euro 500SEL "Blue"
2001 Texas Heeler (Aussie/Queensland X) "Sulphie",
2012 Queensland Red Heeler "Squeak"
Best dogs I've ever had.