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This only happens maybe 1 out of 10 times the AC runs, maybe less, but the buzzing sound is aggressive enough that the neighboring wall on our home vibrates like the hood of an old truck.

On the off chance than the unit is making the extra loud buzz, it is consistently the same loud volume from start to finish. All other times it runs as expected.

I have not noticed this effected the cooling abilities or anything like that. I called an HVAC repairman last year and of course the AC wasn’t acting up that day. He replaced the big capacitor just in case and went in his way.

What could be the issue here?

I’m concerned this could turn into a much bigger problem if I don’t nip it in the bud now.

Short clip of the sound

** EDIT **

  • I'm not exactly sure where the buzzing is happening inside the unit but it sounds like it could be compressor related.
  • We are near SLC, UT so 4750ft elevation and 20f-100f temps during the year
  • This is an original ac unit and an exact match to all the other units in the hoa (builder town-home style condos).
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    can you figure out where the noise happens (fan, compressor, electrical section)? that should help narrow down which component may be at fault. Aug 22, 2023 at 13:12
  • I am not a licensed contractor or HVAC expert, but, honestly, while this may be louder than it sometimes runs, it doesn't sound like there is a (serious/immediate) problem. Are you in an area effected by elevated temperatures, e.g. southwest? Extreme heat toward end of the day might be having a temporary effect...or not. I suppose you could take a ratchet set out there and make sure all the screws and bolts involved are snug to ensure it is not just a loose whatever rattling...it would be a good first troubleshooting step as well as good maintenance practice each year.
    – AA040371
    Aug 22, 2023 at 13:39
  • Good on your HVAC guy for changing the capacitor...this is such an easy and inexpensive task, and caps go bad so frequently (every 3-4 years in hot temp zones), that I recommend to people they keep a spare on hand and just go ahead and proactively change them out every (you guessed it) 3-4 years before they cause a really bad day at the worst time possible.
    – AA040371
    Aug 22, 2023 at 13:43
  • >>Surprised! and Concerned!<< that your condenser unit is not sitting on a concrete pad or other hard, stable surface that raises it above ground level. Is this original unit? When was your home built and where located?
    – AA040371
    Aug 22, 2023 at 13:46
  • I'm not exactly sure where the buzzing is happening inside the unit but it sounds like it could be compressor related. we are near SLC, UT so 4750ft elevation and 20f-100f temps during the year. This is an original unit and an exact match to all the other units in the hoa (town-home style condos).
    – Jacksonkr
    Aug 22, 2023 at 15:26

1 Answer 1

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I've had contactors make noises like that when the solenoid didn't completely seat. It seats enough to close the contacts but not enough to close the gap in the solenoid. It usually caused by loose debris or just wear on the solenoid. Rust can cause it too.

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  • in searching around about contactors (I found some interesting videos)[hvactrainingshop.com/bad-ac-contactor-symptoms/] which may or may not be what I'm dealing with. I'll dig in as far as I'm comfortable and let you know what I find!
    – Jacksonkr
    Aug 22, 2023 at 19:34
  • A follow up on this - if it were a bad contactor shouldn't I hear the buzzing sound EVERY time the outdoor ac unit is running? I'm only getting buzzing maybe 1 out of every 10 times it turns on. When the noise happens it seems to occur later in the afternoon / early evening on very hot days.
    – Jacksonkr
    Aug 25, 2023 at 15:12
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    @Jacksonkr A failing contactor could buzz sporaticly, depeding on the circumstances, moisture, humidity, contact with debris, etc.
    – JACK
    Aug 25, 2023 at 15:52

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