You were right, it was the belt tensioner. The pulley's fine, seems like it's the assembly
For anyone trying to figure it out in the future: 2 ways to find out.
1) remove the serpentine belt. start the engine. if noise is gone, problem solved. - figure out which pulley is making the noise. (spin them around, try to see if anything is loose)
2) to confirm it's the tensioner, you can take a wrench and push on the hex bolt while the engine is running (be careful not to drop it or get it caught up in the belt - you can get badly injured). If the noise goes away, you've found the source.
FYI - you don't need to take off the passenger wheel to properly put in the belt like some youtube videos say, it's actually quite easy to loop it around the pulleys if you stick your hands in there and keep the belt tight with one hand.
I made a diagram for other newbs, it took me a while to figure out what is what!
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