I replaced it less than a month ago with dealer items and it started doing the same thing. Before that I bought off the market belt tensioner and idler and it lasted a short time. Now it squeals as soon as I press the gas and I USED THE SPRAY BOTTLE test and when I sprayed the back of the belt at the alternator it squealed like hell then it stopped if I spray the grooves side nothing happens. So can it be the tensioner and or belt are not good even though new? The belt wobbles a bit on the alt.
Before I did this the older belt left residue on the grooved pulleys and I somewhat cleaned it, I also fixed a hose that was leaking antifreeze on the belt and pressure washed it all. Belt was good for a month now back to squealing.
Need advice thanks in advance.

 

The belt shouldn't wobble at all. Maybe its the pulley on the alternator?
Can you post a video?

 

So I ended up replacing the alternator for a stripped idler thread and a broken contact on the red connector. I put it back cleaned it all with break fluid and checked all the leaks around. So the tensioner and belt are dealer items and the idler pulley is from Amazon, so I put it on and my son drove it to college no noise. 6 hours later he came home and it started to squeal again. I looked at it and no grease on the pulley like before, but I kept the new Continental belt I had before and put it on thinking maybe the BMW one was glazed from the grease that was on before, even though I cleaned it with break fluid. I will see if it squeals tomorrow, today on a short test drive it didn't. WTF is it. Should I get a smaller size belt, don't want to change the ac and power steering for a complete change and fix. I wonder if they have a mechanical tensioner for these situations!

 

Why do you clean things with brake fluid? Is this a common thing?

 

It's a cleaning solvent and since I had some type of grease on the idler wheel it only seemed logical to clean it with solvent. This had oil leak on it and antifreeze from a hose, both issues are fixed now and the wheels need to be free of contaminants as per several different expert opinions on forums. Anyways it doesn't hurt anything since it evaporates really quick.

 

So you mean brake cleaner - not brake (hydraulic) fluid.

 

Yes my bad brake cleaner