Accepting any and all crazy ideas

[highperf4x4]

I have a truck puzzle and I'm open to whatever suggestion flies up on the table. This might take a little bit so bear with me.

I noticed a knocking sound coming from the area of my gas tank the other day which is still behind the seat. I have installed bucket seats so it's wide open in the middle now.

I thought at first it was sound transfer from something like a loose torque converter so I checked it and it's tight. I looked the flywheel over and it doesn't look cracked. I checked the exhaust and it's all tight. It's also new.

So, I thought lifter maybe. However I can't really hear anything under the hood of the truck. It's loudest inside the cab with the doors closed and it really sounds like it's coming from the gas tank. I pulled the oil filler cap and pcv valve and neither valve cover is emitting any noise. By now I'm thinking it's a rod and the sound is transferring through the metal fuel line. It is definitely rpm specific. The trouble is, you can't hear anything under the hood or under the truck. I started it up today and reached down and I could actually FEEL it in the Big fuel line coming out of the tank.

I called my buddy Dr. Joe and he brought his stethascope over and we went to work trying to track it down and here's what we came up with. Nowhere on the engine could we locate a knock with the steth. Not from the valve covers, intake, block, or oil pan. I can't hear anything coming from the fuel pump that sounds like a knock. However, inside the cab, you can hear it faintly from the tank and faintly from the floor of the cab but it's very loud when you put the stethascope on the fuel line coming out of the tank. The "pull" line. I mean very loud.

Now here's the tricky part. That fuel line is insulated by a short rubber line from the tank and another rubber line under the cab and again at the fuel pump. So it's difficult to see how it could be sound transfer. But you can only faintly hear it in the tank and I think that's because the fuel line is laying against the tank.

I did recently change the fuel pump but as I stated before, the stethascope is not picking up a knock from the fuel pump itself or the surrounding block. So, I'm stumped.

Shoot the ideas at me. Keep in mind that it's rpm specific. Meaning it speeds up and down with the motor and when the stethascope is on that fuel line it sounds like a knock.