HVAC problem
Jun 2, 2017 at 10:36am Edited
When our furnace started emitting an alarming noise only when a/c was running (and not when just fan was running), the HVAC guy diagnosed it immediately on hearing it as something stuck in the fan. Can't remember now whether it was a piece of paper or a misguided mouse. He said the fan runs much faster when the a/c is actually cooling than when the fan is just on, hence the difference in the noise.
Took about 10 minutes, a few laughs, probably $50 to $90 for the service call....
This was a fairly steady sound, iirc, and closer to a screech than a hum, so ??
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When AC is activated, a medium pitch, soft cyclical hum is audible indoors at the furnace, and even upstairs indoors. Loudest "at the furnace". This is a change from previous years' behavior. No abnormal sound noted outside at the condenser unit. Delivery of cooled air seems normal.
When furnace delivers heat, or when set to "air circulation only", no abnormal sound noted from the furnace.
Diagnosis?