I like the idea about the serpentine belt tensioner. Another test: while the engine is at idle, using a long piece of wood (broom handle) or long screwdriver, push on the belt tensioner to make it a little tighter, and if you can make the noise change at all, that's the source of your noise.

Other observations:

(1) The plastic fuel line clip at the front of your engine is broken. Left alone, your hard fuel lines are at risk of cracking and breaking. Time to replace all those clips and the little rubber shims under them.

(2) Your little vacuum line at the front of the engine is routed incorrectly, jumping all over. It should be going between the serp. belt and the engine, not on the fan side of the belt.

(3) Other noises: I could not detect anything unusual, but my ear is notoriously bad. These engines have a bad reputation for developing click/clack noises from the hydraulic compensators (aka "lifters"). Mobil1 synthetic oil has a good reputation for quieting that noise, but it may take hundreds or thousands of miles of driving to clear that up.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC