I have a noise coming from the rear gas tank on my '92 F150. A knocking sound when the gas moves something inside the tank. When the tank is more than 1/4 full it is a lot louder due to the echo from the gas. I can still hear it when the tank is empty, or close to it but it is a different sound like metal knocking on metal. Any ideas? I have seen what it takes to remove the tank and really don't want to do that without knowing first if it I need to remove it and open it up.

 

I cant think of what could be in the tank, I would think if your pump came your truck would stop running. did you check your tank straps or maybe your exhaust is loose back there?

 

I have checked everything I can think of. It sounds exactly like something is loose. I have looked, as well as 5 other people. Lifted the truck, run in place, etc. I now noticed that it changes when the rear tank is empty. That's what makes me think it is in the tank. No problems with gas supply using either tank.

 

Your profile doesn't say where you live, but are you in cold country? Did the noise start after things got below freezing? I had the same issue on a car many years ago and it turned out to be water in the gas had frozen into a lump which was banging around inside the tank.

 

i dont know whether these trucks have an anti-siphon screen in the filler necks but a few years back a friend of mine found a hose in his gas tank (not f-series) ... whether by accident or design someone had left a siphon hose in the tank. it didnt make any noise at all but it sporadically interfered with the fuel pump . just sayin' there could be a foreign object in there.

 

Keep in mind you guys, there is no switching valve on 94 and trucks.