It doesn't matter if there is a half tank or a quarter or empty. The only time that it doesn't happen is when it's full.
I thought I read not too long ago something about this?

Normal, it is an indication that your vacuum canister system is functioning properly. Also, if you had a malfunction the OB2 monitoring system would tell you so. Try leaving the gas cap loose and driving the car to see what happens:hehehe:

You would have a problem if you didn't.

Always get big "poof" when taking off gas cap?
It doesn't matter if there is a half tank or a quarter or empty. The only time that it doesn't happen is when it's full.
I thought I read not too long ago something about this?That's A Good Thing as Martha would say.
You can have the cap pressure tested if you feel it's malfunctioning. :z1-2:

I had originally thought that that was only supposed to happen on empty, I guess I was wrong...........again!