Hi! I'm new here, so I apologize in advance if I'm missing information-- just let me know. I live in a townhouse and have a central air unit at the back of the house. It's mounted on brackets roughly at the height of the foundation. I think it's a Rheem (or their parent/subsidiary company). It's been hot this summer so we've run it a lot. In about mid-July, I thought the neighbors must have been drilling or sawing into the wall (the foundation or something because it was a rumbling buzz sounds) but as I listened more carefully, I determined it was the air conditioner. I turned it off and called a repair technician. He asked me if he could hear the sound over the phone but when I turned it on again it was totally normal and was normal again for weeks. It's made this sound maybe a two or three more times since then including today. The trouble is, I can't diagnose it because it goes away when I shut off the unit. I'm optimistically hoping it's just something with the chassis and not with the internals because a loose fan or tanking compressor would be more consistent, no? Anyone have any ideas of what I should do to sort this out? Thanks very much in advance!

Our makerspace has 3 identical HVAC units. Each condenser runs off of a 30 Amp 240v breaker. However, when I check the power being supplied at the evaporation, I read confusing voltage. I checked the black to ground (green to a case ground block) and red to ground. On two of the units, I get 240v on the red wire and 110v on the black wire. On one unit, I get 120v on black to ground and 120v on red to ground. Shouldn't I get 240v between either red to ground or black to ground? Incidentally, two units function fine. On one A/C condenser the fan runs, but the compressor does not. This is one of the units that has 240v on red to ground and 120v on black to ground. Thoughts? Rick