Buzzing sound from Isata 3 stairwell

[RolandRevenger]

I picked up my Isata 3 from it's winter storage site a few weeks ago. After sitting in an unheated building for almost five months, it fired right up. Drove it home, no problems. I was hoping to re-install the coach batteries and de-winterize it, but the weather her in MN has been below freezing; even for MN this has been a super cold Spring. Today, I went out to look for something, and heard a faint buzzing sound. I thought it might be the CO detector, but it wasn't. I had removed the smoke detector battery last fall, so it wasn't that. Weird how sound bounces around in it; finally located where the sound was coming from. The stairwell. I went back to the house for a while and when I came back, the sound of silence greeted me. Could that sound be connected to the solar collector electronics? The coach batteries are still not in it. What sunlight we had today was less when I went out the second time, which makes me think it is related to the solar. Any other ideas?