At Thermostat

  1. Fan was on Auto
  2. Cooling was On

Outside Unit was making noise. Outside Unit Fan not coming on at all

Next to the compressor on the Wall there was On Off Switch, I turned off power from the outside unit but it still kept making that noise


I went inside turned off the Cooling and noise went away

It has two capacitors 30uf and another small one 5uF

Unit is Lennox


  1. Is the issue with my capacitors?
  2. Can i replace these two capacitors with a single 35uf one ?
  3. Or i am thinking it completely wrong and it's something else?
  • can't be certain about pitch and kind of voice, i am very certain it's not coming from the FAN itself, that noise i am familiar with. It's not high pitch but still noticable. What i find odd is i turned off power from outside but noise kept coming – SeanClt Jun 13 '16 at 1:08
  • When you said "turned off power from outside", do you mean turning it off at the mains disconnect for the AC? – ThreePhaseEel Jun 13 '16 at 1:12
  • outside next to the AC Compressor, there is a on off switch on the wall , I turned it off from there – SeanClt Jun 13 '16 at 1:13
  • Changed capacitor still the same sound Hvac supply told me I could change with one and that's what I did – SeanClt Jun 13 '16 at 13:40
  • Yes same situation Hvac supply person told me when I had motor hanged last time contractor added the small one instead of replacing the big one – SeanClt Jun 13 '16 at 13:45

Sounds like contactor chatter. Could be a bad contactor, or not enough power on the coil.

Basically, the contactor is closing and opening really quickly. This happens when the coil doesn't create a strong enough magnetic field to hold the contactor closed.

Depending on the cost of a new contactor, I'd probably just pop in a new one. If it doesn't fix the problem, you'll have to troubleshoot further. If you have (or can find) the manufacturer's specifications for the contactor, you could measure the contactor coil resistance and compare it to the spec.

You can also check the voltage of the control wiring going to the contactor coil.

  1. Remove the low voltage wires from the contactor, and position them so they do not come into contact with each other or anything else.
  2. Set the thermostat so that it's calling for cooling.
  3. Use a voltmeter to check the voltage between the two low voltage wires.

You should measure about 24 volts (assuming it's a 24 volt control system).

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  • Yes that fixed the issue, I changed the contractor $17 Part at local store cashwell appliances in charlotte, NC while changes i felt one of the wire was loose too so it could be the wire or the contractor itself but i went ahead with changing it as i had already bought the part Thank you very much for the help you saved me $200 – SeanClt Jun 13 '16 at 15:50

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