Ford F-150 - 1997-2003 :: 2000 5.4L - Intermittent Engine Knocking Underneath

The knocking started very suddenly 2 days ago - but not after any unusual events, other work etc. Truck has 145 K, and otherwise starts & runs perfectly. Doesn't use oil.

I heard it from in the cab, windows closed. At first, thought it may be a rod bearing - but if so, not terribly loud. Didn't drive it any more after getting home. But starting it in the drive, the knocking was audible.

Then I discovered it * completely disappeared * several times, after shutting off engine, then restart a few seconds later. When that happens, absolutely no knocking at idle or reeving it up a bit. But it may return on the next 1 or 2 restarts, then disappear again.

Since the knocking * completely stopped * on some restarts, does that rule out a bad rod or main bearing?

I haven't seen oil gauge pressure fluctuation, whether it's knocking or not. The gauge is where it's always been - well above 1/2 way. But, the gauge also doesn't move when I rev it to around 2000 RPM (? may be normal not to move, after engine's already running?). When the knocking is happening, increasing RPMs also increases the knocking frequency.

Can't hear the knocking from top side of engine w/ hood open - even using rubber hose to my ear & moving it around. But (when) it knocks, can easily hear it inside the cab or under the truck.

From underneath, knocking is more in the center of truck, near the back of block / oil pan or tranny bell housing. I can't isolate the noise as more on one side of the block than the other.

Could the oil pump itself be making a knocking noise? The knocking is "similar to" a rod knocking, but doesn't seem quite as "hammer on metal" metallic.

There are no DTCs showing on my scanner. Only odd thing was, for "O2 Monitor Test," it showed "vehicle did not return data for any O2 monitor test." Though under the scanner "I/M Monitors," it shows O2 Sens Mon. "OK," and O2 Sens Htr "OK."