What in the world is causing this noise? '03 Ford Focus

I keep hearing this noise off throttle when I’m decelerating particularly. Sounds kind of like the alternator noise is this video but deeper and more bassy.

This next one is mine.

I’ve so far replaced the belt tensioner and idler pulley and have got a new motorcraft serpentine belt in the mail. It’s hard to even hear the noise outside the car to troubleshoot where it’s coming from but I have used a pry bar on the accessories as a stethoscope to listen to them. I can hear the noise in all of them but the loudest and clearest one sounds like the alternator. I just want to be sure before I start replacing things.

The way the noise goes if it’s unclear from the video is when I decelerate to a stop or rev in neutral the RPMs fall to idle and then a second or so later there’s this vibration or reverberation through the car for a couple of seconds before it settles down. This all mostly occurs when the AC is ON but it is still barely audible when it’s off as well.

Can anyone point me in the right direction?

Can you remove the serpentine belt and try it out to see if the noise goes away? That would at least confirm or rule out the accessories, tensioner, idler pulley, etc. My first guess would be AC compressor, but no way I’d replace it without being certain.

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It did go away last time I had the belt off actually. The thing is that the AC compressor doesn’t seem all that loud when I gave it a listen in particular. It’s really strange. I’m pulling my damn hair out trying to figure this one out.

How would I go about ruling out the other accessories?

Get a mechanics stethoscope .


And while the engine idles, place the tip of the stethoscope on each component the belt come in contact with.


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One of my Chrysler minivans had noises that came from the pulley on the alternator. It had a clutch. Replacing it with a solid pulley changed the noise. An auto electric shop put in a new pulley with clutch and the noise was gone.

I don’t suppose the compressor is on a separate belt from the alternator? That would be ideal for testing. Probably not that lucky.

Nope it’s a serpentine so the Alternator, water pump, ps pump, and AC compressor are all on there

Would a long pry bar be sufficient? I’ve been using a 2.5ft+/- one to check it over

Another area to check is, see if the rubber isolator on the harmonic balancer is beginning to delaminate.



I’ll take a look. I believe the sound is coming from the AC compressor now that I’ve had the wife rev it while I listened with a pry bar. I’m willing to take any other suggestions I just hope it’s not what I’m thinking it is. Summer is just getting ramped up here with our first 100 degree day last Friday.

Change the vent selector position so that the AC is off. If the noise is still there, it is the AC clutch, not the compressor, that is noisy. I would still get a cheap stethoscope to verify.

I’ve checked and just the sheer noise and vibration could this actually be the compressor making this racket? On hot days I can feel it in my brake pedal after coming to a stop. I’m kind of wondering if it’s not so much the AC as it is the car not compensating for the extra draw and the engine is lugging a little causing the vibration. I can’t even hear the noise outside the car without a stethoscope.

I checked that from underneath and didn’t see anything that looked alarming. It’s hard to see while running but it doesn’t look like that’s the culprit.

Stop the guesswork… take a trip to your local general mechanic.because he will probably find the problem under 5 minutes.

Originally I had one take a look and he had no clue. I’ve got an appointment to take the car in and have it checked out but that appointment isn’t until the 29th unfortunately. Everyone is swamped all of a sudden around here.

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Turn your steering wheel slowly from left to right,if it changes pitch then it could be the p/s pump or pulley that is causing the noise. I had a Ford that was whining badly back in 1989 and it drove me crazy.

When I had an 05 Focus, the whining came from the front passenger seat. Sorry, could not prevent myself from responding.


Compressor may be dragging the engine idle down because it’s about to lock up. My guess, anyway. Did you ever run the engine with the belt off? I may have missed it.

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Not much noise from the PS pump that I can see. I could hear some noise but it’s not the sound I’m worried about. It’s pretty tame in comparison.

I did give it a try without the belt. The noise vanished without a belt on the car. I’m desperately trying to convince myself it’s not the AC but I’m really leaning that way. Have you ever heard of being able to FEEL the compressor when it groans? I just can’t fathom it making so much noise and vibration that I can hear inside but nothing outside even with my ear dangerously close to the accessory belt.

I may have to revise what I said a little because as I drive it the details become more clear. Today I made sure to keep the RPMs up when I reversed out of then garage and the noise was still present well above idle so I retract that bit about it lugging. The sound is audible as a moan up to 1300-1500rpm and then once the idle falls it trails off in this groan accompanied by a vibration. It really sucks that I don’t know many folks out here that could look at it on short notice.