I can't imagine your timing chain doing it.....but it is getting old. I'd be more prone to suspect the rear bearing on your alt or one of the idlers. Tough one.

Further thought........

For a rattle, it can be a harmonic......but assuming it is or isn't, it has to be a NON-lubricated thing. So rule out the timing chain/valvetrain.

Assume it isn't a vibration harmonic. From that side of the engine, it has to be the crank pulley, the tensioners, the rear bearing of the alt, the water pump, or the AC compressor. Try it with the AC in auto or normal. I'm curious about this. VERY curious. I like the difficult ones

(did I leave out any sealed bearings?)

I believe I've heard it make the same rattle with the AC kicked in. The SC tensioner pulley isn't suspect, it'* been replaced with ZZP'* with no rattle change. The tensioner itself, maybe. I think I replaced the alt with a reman after the rattle showed up.

Remove both tensioner idlers. Oil them with engine oil (let them soak on each side for 30 minutes). Put them back in opposite locations, turned around from the way they were (outside face in on both of them). See what that does.

You mean the pulleys, or the entire tensioners?

Just the pulleys. Tensioner assemblies aren't interchangeable anyway. If one of them isn't true, it'll wear the pulley bearings. If you swap it, that wear will be switched to the other side. And no noise for awhile.

I'll give that a try.