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Community and Q&A

Mitsubishi outdoor unit super loud when cooling begins

JaneG | Posted in General Questions on

I had a Mitsubishi heat pump installed last week, connecting to a (new) gas furnace/ducted system.  I chose the heat pump for the quiet dB rating.  When the system calls for cooling, the outdoor unit is super super loud for about a minute – mid-70s to 80 dB.  Then it quiets down to the usual < 50 dB level.  

I called Mitsubishi and the technician told me the unit should not be loud at any point in the cooling cycle.  My contractor hasn’t “caught it in the act” yet (though I’ve shared a video) and seems perplexed because they’ve had only good experiences with Mitsubishi.  

Does anyone on this forum have thoughts on what may be causing the noise?  I thought it was a semi roaring down the street the first time I heard it.  The unit is under a bedroom window and wakes the occupant multiple times each night.

Thank you.

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  1. scottperezfox | | #1

    When I first bought my house, the AC unit was furiously loud. Turned out that the fan-motor shaft was too long and was grinding into the end of the compressor. The tech cut back the extra shaft length and greatly reduced the noise.

    But something tells me a brand new unit wouldn't have that problem. It's usually something to do with the compressor or the capacitors. Could be faulty parts, or a bad connection on the wiring.

    Sorry I can't help more specifically.

  2. Expert Member
    Akos | | #2

    My guess the charge is off on the unit.

    Have the hvac tech reclaim the refrigerant and weight the charge, you might want to be there to see the numbers so they are not fudging it. Most units have the required weight listed on the outdoor unit, if off make sure they find the reason for it. Recharging a leaking system means the same issue a year or two down the road.

  3. Expert Member
    BILL WICHERS | | #3

    I agree with Akos here, my first thought is also the charge isn’t right. The tech will need to reclaim the existing refrigerant, draw a vacuum on the system, then recharge the system using a scale. This is usually the most accurate way to charge a system, since you know exactly how much refrigerant you put in. The subcooling method is far more error prone than charging by weight.

    Another thing to check is to make sure the refrigerant lines are not forming a rigid connection between the compressor outdoors and the wall of the house. Assuming a ground mounted outdoor unit, you want all the refrigerant and electrical lines to make a “U” bend before entering the home, you do not want a straight connection that can rigidly couple vibration from the outdoor unit to the side of the house. If there is a rigid connection here, the wall of the house will act to amplify the sound.


  4. JaneG | | #4

    Thank you, everyone. I'm grateful for your help.

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