Its normal, all cars do it, usually its a bit worse in hot weather too. Its the fuel vapours that have expanded in the tank and when you release the cap it makes a hissing noise as they escape. A lot of cars have a valve in the cap that allows air in but not out...at the workshop where I work we have to test Jap import fuel systems. Must hold 5Kpa for 6 minutes...if your hearing a hiss, it means the fuel system is in good order.
yeah, mine does it as well. but a word of warning here - never stand directly in front of the cap when you release it. The hissing sound isnt a problem, but if it gets any stronger fuel can actually come out.
On the weekend up at mallala we went to refil dads sprint before my next track run. As we cracked the cap a small spalsh of fuel came out. But nothing compares to earlier this year when we were travelling across to Bathurst to compete in a hillclimb. It was 42deg C, and at the town of Hay in the middle of the Hay plains we stopped to refuel. As dad opened the cap it shot out of his hand, and several litres of fuel came gushing out at the thickness of the filler nozel and doused my dad in several litres of 98 octane! Sure it was abnormally hot and after putting the 16V engine into the sprint with a different fuel system we hadnt quite got the venting system sussed out completely, but still be very careful as this can happen!