Today when I was in the line at a drive through, engine loping at idle, there is a faint knocking. It's been doing it for some time now. Today listening while it was doing it in the drive through I listened really close and it was absolutely rock steady, no change in rhythm except if I stepped the rpms up a little, and then it just sort-of ... went away. I've tried poking around with a stethoscope, long stick, etc several times. I had about decided was a loose timing chain slap but I can't hear it in the stethoscope anywhere on the engine near the timing area, It never is heard at above idle. but I have never tried the fuel pump At idle, I have 40+ oil pressure. I had also checked the timing chain when I was doing bearings last year. It's been bugging me for awhile, and listening today, it hit me.

The fuel pump on the 351M is causing the low idle knock.

I know what rod or main bearings sound like, it's neither of them ... and they get worse at higher rpms. It was partly why I did those bearings.
First time for me. Anybody else?

I'm home now, supper is done, now to go get that stick.

Never mind, about 99.44% figured out. I was close, but not what I thought. Nope, not in the fuel pump, rather I'm pretty sure is the actuating arm eccentric after careful analysis. I listened to everything I could get a long screwdriver or my stethoscope to reach, at no minor risk I might add. Engine idling, in low range to really drop rpms like would be in a drive through, parking brake set, & tires chocked. It is not timing chain, and not the fuel pump either. I am 99.44%sure it is the outer ring of the eccentric being slapped by the inner ring which it fits pretty loosely around. It actually stopped after I guess it got enough oil spilled out of the lifter galley onto it and the cam sprocket. I have a spare / extra / leftover eccentric set in my parts storage. The OD of the inner cup like is 2.368" .... and the ID of the outer ring that rides on it is 2.418" ..... so there is about 0.050" free room between them.

Been hearing it for years but only ever when idling really low in gear it seems. Internet search tells me I'm not alone, but knowing pretty certain what it is, no worry no more.