Just noticed recently that when I go to fill up, there is a clearly audible suction noise as I take the fuel filler cap off. I'm guessing this is caused by a vacuum developing in the tank as the engine draws fuel. The engine runs fine by the way.
Is this normal or is there a breather somewhere that could be blocked? I've had a good look at the cap but is doesn't look like it has a breather built in.
Any help appreciated.