humming sound

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?

I can’t help with your query, but if I were in your situation, I would call Trane directly and speak to their peeps…

Bruce if a stat is calling for cooling or what ever mode the stat is set for the exterior contactor if going bad such as burnt points not making good contact will hum excessively any service tech that knows anything would be aware of this.
To vertify this all one would have to do is turn the Thermostats to the off position and the exterior humming will stop.

I have on many occasions taken a screw driver and applied pressure to the points on the contactor to stop the humming.

I always write this up as contactor making excessive noise replace as necessary

Not sure if you remember Charley the old wood stove combo would do the same thing.:mrgreen:

I know, but the noise is there even when no t-stat is calling for cooling or heat.

There is nothing out at the unit that can hum except the contactor. I would have pulled one of the 24 volt wires off of the contactor. You can usually see the points on these contacrtors as they are open faced would be rare to see a enclosed contactor.

Sorry I re read your post it was a HP the reversing valve has a 24 volt load that can hum also

Hit the contactor itself with a hammer usually the hum goes away.