My 2003 Altima has a low frequency rumbling sound when idling. I joke it sounds like its running under water. Its not noticeable at higher revs. The sound seems to be resonating along the fuel lines. The noise seems to be somehow transmitting into the bodyand is heard on the drivers side, and even louder near the floor. All fuel line mounts are intact. I suspect the fuel injection system is responsible...Does not sound like the tick tick tick of a manifold leak. Any suggestions?