+1, mine will slosh around a make a similar noise when I first park it after driving it.

 

I have a tennis ball in my glovebox.


Will get some microphones around the car soon. I did get under and look around and could not locate anything accept a basic location of driver rear wheel area.

I have duals. Also, the sound continues for some time after the car is off so I doubt it's the tank. I uncapped it and listened inside the tank. Not from there. The exhaust is the original thing I thought of, but still not entirely sure. So far it is my best guess.

 

def. haunted lol..check to make sure everything is tight under there. check the bolts and nuts that hold your fuel tank in place..if your fuel is sloshing around and one of those is loose, maybe it's rocking the fuel tank a little, one of mine was a little loose and I had the same knocking (I think). Just my two cents...who knows..either that or you just have a mustang ghost.

 

As of now I am still going with haunted or exhaust expansion/contraction. Everything is tight underneath. It was on a lift in December and checked. Maybe the previous owner died and wants it back.