I have attempted to search for this issue, but I have been unsuccessful.

I am new to this forum. I just bought my first Blazer for a pretty good deal a few weeks ago (1998 Blazer LS 4x4 4-door). Was a single owner, extremely good condition overall, had a few minor issues when I got it and I have been able to figure most of them out myself.
But this one I cannot figure out and have searched and searched. So here I am...

Every single time after driving around for a while and parking, shutting off the engine, within a few minutes there is a loud thunk. It sounds like it's coming from the rear, around the gas tank. Doesn't matter if it has a full tank or an empty tank.
It has an 18 gallon plastic tank.
I put a new gas cap on, thinking maybe it wasn't venting properly. Didn't help.
I have been trying to find a new vent for it. But I can't find one that's for the 18 gallon plastic tank.
I have a new fuel pump ready to go in. Mine is pretty loud and would rather replace it before it fails while out on a camping trip. But I doubt the pump is causing the thunk in question. But when I have the tank down I will try to test the vent.

Has anyone else had this issue? I am at a loss.

Thanks

 

Not an expert on this but I'd try to cause the thunk. For instance. You might try removing the gas cap immediately after you stop to see if that causes the thunk. You might also tap on the gas tank to see if that causes it.

 

well I have been removing the gas cap after driving and thought that had made it stop. But today it made the thunk even with the gas cap off. I would assume that means the gas tank venting is not an issue

 

muffler

 

Yep. That would be my conclusion too.

 

Because of thermal expansion and then contraction of the exhaust system?

 

that’s my next guess. Tapping on the muffler in certain areas makes a similar sound