It sounded like something that can happen when overfilled. At 3/4 full, that wasn't it. Did you check for any cracks on the tank that are not visible at first glance? If it happens again, you can send it back so we can take a look at it, without mailing charge of course.
I will check for cracks later, I don't think I will need to return it, if it was doing this every time, maybe but this is not enough to worry about, I just wanted to ask if someone else had it happen is all.
KAS