anyone had this before? only does it when i fill it up with fuel...... Anyone know what it is?

 

Just sounds like air releasing from the tank.. work vans do it as they top off, but it sounds lime someones emptying a bath

After you fill it up, try not to tighten the fuel filler cap, just put it on. Maybe this will help the air expand. 

I was thinking it may be creating a vacuum, as it sounds like its gulping air in....

come to think of it, sounds like a heartbeat! lol...