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Alternator or compressor

Can’t figure out witch one is making noise

Use a piece of hose as a makeshift stethoscope or,
Take the belt off & spin the pulleys by hand.

ou can use a piece of heater hose like a stethoscope to try and pinpoint the location, beware of moving parts.

The hose idea as posted above is what I’d try first if I had that problem. If that didn’t narrow it down, what I’d do next, I’d put a long wooden dowel, like an old broom-handle, on the possible culprit and listen at the other end, the sound will transmit up the dowel, and sometimes that method can narrow down the sound source better than the hose method. Sometimes I’ve thumbtacked a paper cup to the end of the dowel, forming an earpiece, makes it easier to listen to the sound. The removing the belt method and hand spinning the pulley often is what works best, but involves more effort.

Witch one is more likely to make noise?

And thanks guys…prob try taking off the belt mostly cuz I don’t have anything to listen with (was thinking about doing that btw) but also cuz I’ve removed the fan shroud twice already to tighten/replace belts…I think I may have possibly caused the noise by tightening an old belt…funny thing was it wasn’t all that tight…new belt now,still noise…was just driving for awhile and hadn’t made the noise in the last hour of driving…weird…hope it’s not the compressor…

The noise could be coming from any part driven by that belt or any belt if you have more than one, compressor, alternator, idler pulley, tentioner pulley etc… If you loosen the belt you can spin each item by hand while feeling for any drag or roughness and listen for grinding. You don’t need anything special to listen with except your own ears.

What is the make, model and engine we are discussing???


It could be either of them, or neither. What kind of noise are you hearing? Is this a 2003 Subaru Forester, as your screen name implies, or something else?

You may be able to tell by a temperature difference. The bad pulley would run hotter. I’d start it cold and let it run for just a few minutes, before the heat from the alternator and other parts hit the pulleys. The heat from a bad bearing would show itself in minutes.

I recently purchased an IR temperature gun for $14 from Amazon, nice gadget and it would be really handy for this type of situation.

AC compressor? Does noise change with AC on or off?