I have lived in the same two story house for about 7 yrs, and this noise is new. The first time we heard it, it was like an electrical zapping noise, heard only behind a wall beside the furnace and hot water tank downstairs. It was loud enough to be heard by my daughter in the basement livingroom, about 10+ feet away, and it was getting louder over a few days.

It sounded like electrical 'zapping' to me, coming from behind the furnace area, but it seemed to be in the enclosed wall. I called a furnace guy out, and he replaced my furnace switch as well as cleaned the furnace (but not the ducts). Coincidentally I had to replace my hot water tank within a day of that, and then I didn't notice the noise again for several days.

Suddenly the noise start appearing in my upstairs ensuite bathroom heating duct, and it was loud like before. I can hear a hum, similar to the sound of the furnace when it starts, but it isn't starting. Randomly I hear a 'fluttering' sound, like a big moth is trapped. I also hear zapping though too, like an electrical short.

I've gone around to every other vent, and hung around in the basement to see if I can hear it there (while it is going on upstairs). I can't hear it anywhere else ... not even in the vent in my bedroom (about 6 ft from the ensuite) or the main bathroom (about 6 ft the other way).

The only time I notice it, is in the wee hours of the morning, i.e. between midnight and 6:00 am. The house is almost always very quiet, all day, and the only thing that might be different then is that bathroom light is left on during the evening till I go to bed (between midnight and 2:00 am).

I have gone in many times during the day, and even spend hours in the room during the day time sometimes, but I never hear that noise. If I turn out the light, the noise doesn't stop right away (I usually fall asleep before it does)... but if it is bees or bugs of some sort, I would imagine that it would take a while for them to settle down anyway.

NOTE: most bugs are down for the season by now, but it has only gotten to freezing in the last few days. I would think bugs would be dead without access to the outdoors by now though ... it's been almost 3 weeks since it started.

I can't imagine I can shoot some "Raid" down there, even though it is probably 20 or more feet to the furnace, eh?

It really sounds like electrical zapping mostly. Any ideas?

Thanks!