Our thermostat regulates AC/heat pump. Early this morning, it started
buzzing rather loudly while on the "heat" setting. It had been on the
same setting all night and started buzzing spontaneously. I thought it
might stop on it's own, but after about a minute I adjusted it - thought
I had moved the button to "off" but it was dark and I had switched it to
"emer heat" which stopped the buzzing. It's never done this and I am
puzzled as to what might have caused the noise.

On Jan 30, 7:32*am, "
wrote:
Our thermostat regulates AC/heat pump. *Early this morning, it started
buzzing rather loudly while on the "heat" setting. *It had been on the
same setting all night and started buzzing spontaneously. *I thought it
might stop on it's own, but after about a minute I adjusted it - thought
I had moved the button to "off" but it was dark and I had switched it to
"emer heat" which stopped the buzzing. *It's never done this and I am
puzzled as to what might have caused the noise.

Does it have an internal battery or does it derive the internal
battery from the 24V power system?