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We got a ductless heat pump system installed in the spring -- a four-zone ourdoor unit and four separate air handlers in different rooms in the house, all Mitsubishi.

We didn't notice any problems through the summer, but after switching to heating we've noticed that one of the air handlers regularly makes a popping/clicking sound, and occasionally a hiss.

At first I figured I was just over-attentive because it makes these sounds while we're going to sleep, but now I'm pretty sure it's the only unit that makes that noise.

How concerned should I be?

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    I assume it is still under warranty, I would Contact the installation contractor and have them repair it under warranty. That noise should not be there. Do not sign off on it until you have used it at least a week and it is ok.
    – Gil
    Commented Dec 4, 2022 at 21:56
  • Assuming it was put in by legit Mitsubishi installers, there's about 11.5 years remaining on the warranty, as best I recall.
    – Ecnerwal
    Commented Dec 5, 2022 at 1:21

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Probably defrost related. If it switches the louvers to the upper setting and blows cool air at the same time it makes the noises you notice, very likely defrost related.

When in cooling mode you don't have defrost events, so no strangeness there.

In heating mode, the units switch to cooling mode every once in a while (depends on outside temperature and dewpoint) to melt frost on the outside unit coils. Depending on outside unit there may also be a resistance heater to speed the melting. What you are hearing is the reversing of the heat pump from heating to cooling-for-defrost, then back to heating.

But check with your installer to be sure it's in the realm of "normal" rather than "needs fixed." Since I use my heat pumps about 99% for heating, it's something I'm quite familiar with. I don't know if the defrost cooling would only be on a single head of a multi-head unit, as my two are both single head so I have two separate outside units.

Also - if you haven't washed your inside unit filters, you're about due. But that has nothing to do with the noises.

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