My mom's 245 had some noisy belts, so I changed the p/s and compressor belts. Turns out it's the alternator/water pump belts, which are of course a bigger pain to change. So as a temporary measure, I sprayed some STP "belt dressing " on them and tightened them a bit. Now there's this high pitched whirring sound coming from around there, gets louder with revs, and loosening the belts doesn't seem to have much effect. The alternator is "brand new"/ remanufactured Volvo part only a few months old.

Is it possibly just the belt dressing, until it wears off a bit? Does a problem alternator make noise like this (like I messed up the bushings or something)? I don't get it.

When the alternator was done, did you/the shop do the rubber bushings? I'm guessing that this is your noise. you should be able to see the belts turn outward toward the radiator as soon as they come off the waterpump pulley if the bushings are shot. Depending on just how rebuilt your alternator is (hopefully they replaced the bearings), the bearings in it should be ok.

It's likely the belts are still talking, but you never know.

You need to isolate this sound. The simplest way I've found is to aim a fine stream of water (like from a plant mister) at the various belts and pulleys until the sound goes away, or at least changes. You'll be amazed how well this works. You can even try shooting a little just behind the compressor pulley in case the A/C clutch is the source of the noise.

I'd be willing to bet a warm Guinness that when you tightened the belts you've forced the pulleys to be more misaligned with the fan and crank pulleys. You can check this with a straight edge. This happens because the rubber bushings used in the alt, p/s and compressor mounting brackets are easily compressed when they get old. Easing off on the belt tension may help a bit, but then you're inviting more belt slip so the proper remedy is to replace those bushings. Belt dressing is at best usually just a temporary solution. The bushings are all the same size so just count how many you need. I've found the alternator is usually the first to have the mounting bushing problem, but if you plan on keeping the car for awhile you're best to do them all. If you're planning on selling the car tomorrow then you can try rotating the existing mounting bushings and shimming the mounting bracket until the pulleys are re-aligned.

The dressing is acting as a lubricant, allowing the belt to slip more than it did before. The original noise may have just been glazed sides which makes the belt "creak" as it goes into and back out of the pulley groove.

If the car has a voltmeter on the dash and shows proper charging voltage (the light won't help here), then turn the radio up until you can replace the belt.

If you can't be sure, change it now. Don't forget to eyeball the alternator for proper position before you take it all apart. If it's offset or tilted, replace the pivot bushings (only about $6 from Volvo) or the new belt will lbe noisy soon too.