[masraum]

Originally Posted by 911 Rod

I thought it was just me. Very mild, but it sounds like a diesel idling outside a few doors down.
The first time I noticed it I asked my wife why a truck would be idling when I'm trying to sleep. She's like "uh?"
I've never mentioned it to her again.

Until reading some of the responses in this thread and perusing a few websites with audio, I had no idea that it could be low frequency (although if I'd thought about it, I'd have realized that any or multiple frequencies were possibilities), and anything other than constant. Mine is a pretty consistent high pitch whistle. Any variation is, I think, me/my brain tuning it out. I have no rhythmic pulsing or thumping.