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 1st, 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."

Suggestions for next diagnostic steps? Thanks.

 

Listen close in the Cat converter areas.
Broken ceramic sub-straight can do/ sound like that and be intermittent.
Good luck.

 

Thanks Bluegrass.

Since the knock randomly stops completely - on some restarts, does that rule out a bad bearing? (but may NOT confirm or rule out intermittently malfunctioning oil pump).

Yes - I listened w/ ear next to both Cat Conv's... and the oil pan, and the tranny pan and lower block. Both with & w/o a hose or tube, next to each area. Actually, several times in all areas (but I may be a little dense).

How likely is it both CATs would begin making this sound, at the same time? And when the knocking happens, it's relatively consistent pattern (not, knock regularly a few beats, then quiet 1 - 2 seconds, then knock regularly, then not...).
When knocking happens, the frequency ("beats per minute") increases & decreases directly with RPMs. Easy to hear from inside cab.

There is some noise coming from the converters. Which sounded "similar" to the knocking. In my limited listening (so far), I couldn't say, "Ah! The noise is definitely louder right by the CATs." I may need to put an ear plug in one ear, & listen w/ the other. I have a medical stethoscope - may / may not help.

And IIRC, the "sound" from both CATs was about the same type of noise & about same intensity.

I can jack it up to give more room, but yesterday (on a creeper) I basically had either my ear, or a 5/8" hose, or short piece of PVC right next to each component.

I've listened to a few vids / audio, of CATs making rattling sounds, but I'd guess the exact sound would depend on what's actually wrong inside them. Of those few audios, most sounded more like somewhat inconsistent rattling, vs. fairly consistent "knock, knock, knock."

Is it possible that on several starts, the oil pump could put out really low pressure, causing some bearing(s) somewhere to knock, then on other random restarts, everything is fine?

However, that would mean my oil gauge isn't showing any pressure change (unless I totally missed the needle dropping, during times of knocking). Can check again - but I watched the oil gauge multiple times.