8 yr old Trane 3.5 ton heat pump with 3 zones (all digital t-stats).
Exterior unit makes an excessive loud humming sound even with the disconnect pulled. (Something on the low voltage control circuit)
I reported it as needing investigation and repair.
The buyers agent called and said the sellers HVAC guy told them it was a rheostat for the zoning causing this and it was not a problem.
I can believe it is not a major concern but don’t think a zoning component has anything to do with the exterior unit. I also doubt an electronic zoning system has a rheostat.
Anyone have any technical info to back up either opinion?