Gonna get some SeaFoam and try it first.

Ordered a harmonic balancer today. If the Foam doesn't work then I will have the balancer on hand to swap it out if necessary.

If it's the harmonic balancer you will be able to move it back and forth by hand, the rubber deteriorates over time allowing the play it's more of a clanging noise than a knock.

It is more like an inconsistent rattle, like a loose bracket. It goes away with any sort of revs and smooths out when the A/C unit kicks in.

It's probably not the harmonic balancer the noise would speed up with engine rpm it may be the compressor???

That's was my first assumption. Maybe I can take the belt off and let it run for a short bit.

I know there's a few weeks on this post, but i just went through this - had a "rattle" in the front of the engine at idle and low speed, out of the blue. Started peeking around under the hood with the engine running and noticed the balancer had a wobble to it, so I replaced the balancer and the pulley in front of it. Truck was noticbly smoother running and quieter, but the noise was still there - it almost sounded like a heat shield rattling at lower RPM and nearly gone at the higher engine speeds. That turned out to be the vacuum pump starting to go. Still had vacuum, but was dying. Replaced it, and now all is good.
The bad part is that all these bits had sort of failed in unison. If the pump hadn't start to whine, I never would have noticed the balancer being out of true and that could have gotten pricey. I would slip the belt off the engine and run it to check and see if your noise goes away - you might be able to isolate what is is. Or, you might have fixed it and put 1,000 miles on it by now...

Nope. Just got two cans of SeaFoam through Amazon. Half price than OReillys.

Already got the balancer but haven't installed it. It is my work truck, so I have to schedule time for maintenance.

There is an annoying squeal coming from the vacuum pump area and I have suspected it needs replacement for some time now. However, I plan to get a programmed ECM and a mechanical wastegate do just do away with the vacuum pump, EGR, and the bank of crappy solenoids that control them.

Found the problem.

Saw fresh metal on the bolts of the balancer pulley. They were making contact with the "arms" connected to the outer ring of the balancer pulley. The composite had sheared from the inner ring completely.

This is why it went away with revs and why the rattle became steady when the A/C kicked on.

NAPA came through with the part within 24 hours. I removed the belt, shroud, and fan. It took an hour to remove and replace.

Now peace and quiet.