Problem: I ran our heat pump, air conditioner all night and it did great, but when I went outside this morning the unit was making this buzzing/humming sound. It still works correctly, but it makes that humming noise even when the unit is not running. In other words when the thermostat has the unit in the off mode it makes the noise, but when the temperature in the house rises and the thermostat turns the unit on everything is fine.
Answer: Many heat pumps have a reversing valve solenoid that is energized (24 volts AC) all the time in the cooling mode. This reversing valve solenoid is energized whether the unit is running or not. If the solenoid is getting old, or if the solenoid is out of alignment, or loose then the buzzing, humming sound will occur. Try to switch your thermostat to the heating mode and see if the hum is still present. If the hum is still present then you might have a noisy outdoor transformer. If the noise is so loud that it is bothersome then you may need to replace the reversing valve solenoid or transformer.
Another thing I forgot to mention. You might want to test your transformer and make sure you are getting 24 to 28 volts AC out of the secondary on the transformer. If the transformer is producing low voltage (under 24 volts) then this would cause a loud humming, vibrating sound. Please click here if you would like to see the low voltage transformers we sell. Below we have a Youtube video made by Supercool Slike Rules which shows how to test a reversing valve coil. Thanks to Supercool Slide Rule for making this informative video! We also have another video made by grayfurnaceman which shows how a reversing valve works. Thanks grayfurnaceman for make this informative video! Please make sure all the power is turned off before testing or inspecting your air conditioning parts. If you have any questions please email as anytime at: email@example.com or comment below. We would love to help you out and have your business!