1999
Buick lesabre, 100,000 miles 3.8 non-blown

Anyways, sometimes when driving or idling in the parking lot at work [to keep the heat going] I hear what sounds like something popping [like a ballon] from the back.
It is not super loud, and the car does not need to be driving to happen, it needs only for the engine to be running.

One other thing - it happens only when the gas tank is not full.
Now this may sound a stretch, but is it possible the fuel pump might be sparking or shorting out now and then and igniting the fumes in the gas tank? As we know, liquid gasoline will not readily burn... I don't know if such an ignition would be forceful enough to detonate the gas tank...

The car runs fine, there are no codes, it uses a normal amount of gas, and there don't seem to be any fumes. The cap seems fine. Only the occasional pop like a ballon.

I know this sounds REALLY stupid, but I would rather sound stupid than be BBQ.

I am thinking try and listen outside or underneath. Maybe put a camera or something under there to record while you have it idling.

Agreed. If you dont want to do that, replace that pump and check the wires. Hopefully it will go away.

Instead of changing parts..let'* test first.

Can you test the fuel pressue?

Had a similar noise myself. Sounded like the tank was "popping" Like an expansion/contraction kinda thing. Usually after driving for a few minutes. I don't hear it anymore and didn't realize it till you brought it up. I was thinkin strut mounts at the time. Wish I could tell ya more.