I've got a vacuum developing in the fuel tank which releases with a 'hiss' everytime I open the fuel cap, and occasionally I can hear glugging and air movement noises whilst I'm driving. This is presumably the tank trying to suck some air in when the vacuum gets too much.
I remember ages ago on here somebody saying something about a breather that stops working or gets blocked, but I haven't been able to find anything by searching the site.
Can anybody tell me what is the most likely cause of this and how to put it right please?
Very grateful for any advice.