Engine Serpentine Belt Rubbing Near Tensioner

[Eilermoon]

Hey all! I've had a squeal that's progressively gotten worse and lead me to replace the idler pulleys recently. I took another look with the belt off and can obviously see that it's rubbing the engine block when it's running over the tensioner, as in it doesn't stay on the tensioner enough and moves toward the block. It's clear this is the issue because I can physically push the belt away from it and the squeal goes away in addition to the blackened spot on the block where it has contacted. Any idea what causes this?

My initial reaction would be a bad tensioner but I replaced it last fall, is it still likely I bought a bad tensioner? I can also notice the tensioner jumping around quite a bit whole running, I'm not sure how much is normal. As a sidenote, I did the 'water test' and sprayed some on the ribbed side of the belt; the squeal went any immediately. This lead me to believe the only issue is this rubbing, caused by a faulty/deformed tensioner or misalignment in some way. Though I didn't see any misalignment when I looked for this briefly.

 

[dr.zed]

On my 98 that tensioner you talk of doesn't move around at all when running, maybe minor movements but certainly not what you describe...

My vote is bad tensioner (again!).

 

[Eilermoon]

On my 98 that tensioner you talk of doesn't move around at all when running, maybe minor movements but certainly not what you describe...

My vote is bad tensioner (again!).

So I ended up emailing the company I bought from originally and they sent me a replacement for free! (1A Auto for a quick plug, their customer service is amazing). I threw it in 2 days ago and seemed fine at first but I did still notice a lot of movement in it. I also noticed more squeaking this morning but haven't investigated further, so I'm not sure that fixed the problem. I'll try to throw a video of it in here after work today.

 

[Eilermoon]

Changing the tensioner definitely didn't fix the problem! Got fed up today messing with the tensioner and realized just how much it was bouncing around, so I concluded the tension or sizing of the belt has to be wrong. The belt I bought only back in December or January is a ACDelco 6K935, which seems to match a 2003 4.6L just fine according to the internet at 94". I decided to buy a new one from Autozone at 93.5" and change it in the parking lot over my lunch break and the difference is night and day!

Video links:
Before belt change
After belt change

So I guess my belt was just slightly too large, causing it to move on the tensioner and rest against the engine block. Getting a tighter one fixed the issue entirely.