When I have had a constantly noisy belt, I used a squirt bottle and sprayed plain water on. If the noise goes away briefly, I would first check tension and belt condition. They are usually OK! Can't tell you how many used belts I still have that look in perfect condition.

I too bought belt dressing in a spray can (Canadian Tire). Very inexpensive. It doesn't actually spray, it discharges a thin long stream of dressing. This makes it easier to get it to where you want it. Once applied, I found it worked for at least 3 or 4 months.

If belt only squeaks on startup, I would check tension. Not too loose and not too tight. Then give belt some dressing anyway! PS often was culprit on my old cars.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5