M20 "screeching" at 4k rpms

[BMWManiac]

If I coast up to 4k rpms, I hear nothing, but if I'm applying load to the engine, like clockwork, once I hit 4k rpms, I hear this high pitch screeching....I am not sure if it is a belt or a pulley....sometimes happens on cold start ups but barely briefly. I tried rev'ing the engine but I can't replicate the noise.

Belts were replaced in June 2013 and have 11k miles on them.....I plan to remove the A/C belt first since I don't use A/C. Next, I will check the other two belts for fraying/tension....any typical culprits?

1991 325i w/ M20; engine completely rebuilt in June 2013, all new belts, water pump, timing belt. I didn't change any of the pulleys or tensioners

[ahlem]

Belt tension is easiest to check. If you are starting to have an alternator or PS pump go out, it could show up as added resistance at the belt. You can try belt dressing and a small bit of lube at the shafts only on the alternator and PS pump to see if that creates a short term solution. You can also use a stethoscope on the alternator and PS pump to hear bad noises.

[BMWManiac]

I checked belt tension, all seems well.

The only thing that seemed sketchy to me, is the alternator bracket to the motor....when i pushed onto the alt belt, the entire alternator slightly moved....I guess I need to tighten down the bracket.

[jeffnhiscars]

It may just have been the early alternators, but there is a rubber mount bushing that once it goes soft, will throw the belt off line, possibly only at speed. There's a urethane replacement that iirc I got from bavauto

[BMWManiac]

Yes, I researched it and the later alternators dont have one. The bracket that applies tension and connects to the engine....is it supposed to have any flex or feel pretty solid? Im guessing solid to keep ension, but I also know that piece of metal is thin. I will look to tighten that bracket and goe from there.

[jeffnhiscars]

The alternator should not move. If it's flexing something is out of whack.

Have you check that the belt is in line with the crank ?

[BMWManiac]

Ok, I haven't had much time to take a look at this, but now I do....

The alternator bracket was fine, not sure what I saw, but there is no movement. I'm going to remove the belt to the compressor first and see if there is any change. I'm assuming this noise is caused by a belt out of alignment? Next thing will be to check power steering pump

[EatsHondas]

It's most definitely a belt. My alternator/water pump belt let me know just the other day. I turned on my electric fan and a loud screeching noise started immediately (more alternator load, belt starts to slip). It didn't stop until I turned the fan off (and later tightened the belt properly) and was rpm dependent.

The noise is caused by the belt not getting a grip on the pulleys. Instead it slides and chatters over the belt surface. This happens on extremely worn belts, or good belts that are too loose.

[BMWManiac]

My belts are 2 years old, have 10k miles on them....I'll check to see if the noise is there with my fan on as well....I think it is either my A/C belt or power steering belt because the other day I drove through a large puddle, and without any load, I heard some screeching....either way, i'll remove the a/c belt, check it out, and then replace the other belts if need be.