I live in a multi level condominium townhouse, with electic furnace/hot-water tank and security alarm bell in basement. When I got up this morning, I faintly heard something and as I went down to the kitchen level and went into the kitchen it got louder. Not horribly loud, if I noisily run water in the sink, it masks it. At first I wondered if it was the neighbor's monitored security alarm. (My neighbor is out of town for a prolonged period). I went outside, and even put my ear against my neighbor's door, but all was quiet. Back in the kitchen, I unplugged everything that was plugged in except the fridge and stove since they are hard for me to pull out of their niches, and didn't seem to be the source of the sound. No change. I put my ear against the forced air outlet on the floor, but the sound didn't seem to be emanating from there either. I would appreciate any thoughts, tips, or suggestions! Thank you!