https://youtu.be/GrowRbDBSfg (15 second clip from inside the vehicle).

Some notes:
1) I can only hear it from inside the vehicle with the doors closed, as the other engine noises otherwise drown it out. However, I have not had a chance to try to listen with a screwdriver up to my ear.

2) It is definitely loudest seemingly from behind the driver's side dashboard area.

3) It only occurs when engine is at full operating temperature.

4) It started occurring about 1500 miles after my last oil change with Mobile 1 5W-30, and concurrent a timing belt / water pump / tensioner / thermostat / radiator change.

It sounds "softer" than every instance of rod knock I have ever experienced, but I have heard some grumblings about a main bearing issue? My other thought is an exhaust leak from the manifold.

002 with 136xxx miles.

Thoughts?

Thanks!

Does it seem rpm dependent? Hard to tell from the video?

Ah, yes. However, it seems to stop abruptly above about 1200-1300rpm. It also only does it under idle and lighter loads (as evidenced by the increasing intensity as I lightly rev it).

I don't think rod knock is very common with these motors, and it doesn't sound like it to me. Is your fan blade broken or something else that rotates that might smooth out at faster speeds so you don't notice it? You said you just did the timing belt, is the harmonic balancer on tight enough? That bolt is very tight, maybe it's loose and has a wobble to it? Not really sure, just throwing some ideas out there. Hope it's nothing serious.

Hrmm. Good question on the fan clutch. I do feel like I can very vaguely hear it when I get under the truck behind the driver front wheel, but it is so faint it might genuinely be my imagination heh. Everywhere else, though, it is definitely too loud to hear anything over all the other mechanics of a running Toyota V6 :P . I will see if I can get in there this weekend. I did read that some folks alleviated apparent knocking by slightly loosening the drive belts, so I might try that as well. I am also considering running 10w40 for a few hundred miles just to see if that changes anything.