This is pretty interesting because I've been hearing a low-frequency hum for the last few years -- since moving into my current home. I only notice it at night,

This is pretty interesting because I've been hearing a low-frequency hum for the last few years -- since moving into my current home. I only notice it at night, lying in bed when it is quiet. To me, it sounds like an idling car sitting within a few hundred feet of my house. At first, I suspected cops were sitting near my house (on a corner, near a stop sign) waiting for people to run the stop sign, but I never found any idling cars when I went to investigate. Fortunately I'm not bothered by it.