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I have an air conditioner that has an in-floor intake. When the heat pump turns on, there is a fairly loud humming that comes from the intake. If I step in certain places around the intake the humming will go away until I move where I am standing. The ducting is in the crawl space under the house and actually where I step to make the humming goes away is in the opposite direction of where the ducting runs by a couple feet.

Does this mean that part of the ducting is loose under the house or something and I just need to find the spot and tighten it down?

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up vote 2 down vote accepted

The usual culprit is resonance. Your pump vibrates at a particular frequency, and while the mountings are usually damped using rubber bushings enough vibration passes to the floor/wall that anything else nearby that resonates at that frequency will vibrate if not fixed securely.

Ducting should be your first check, but also floor boards, drywall panels, or anything really.

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