There is a beep noise in the house that only happens every couple hours or so and I have no idea where it's coming from! π
Loud or faint? Smoke alarms are usually louder. If more faint, a phone (maybe an old one?), laptop, old ipod somewhere, wireless headphones, maybe a battery back up?
I kept hearing a weird noise while in my dining room. For MONTHS. Finally traced it to the husband's computer in the room overhead. It had a loud-ass harddrive, and every time it kicked it the sound would run through the ceiling! But in the room with the computer it was barely audible.