Does he have a computer running something in the background? Maybe Seti@home? I’ll bet it’s the aliens. They’ve been on hold since 1971, they’re out of patience, and their enormous battlefleet is on the way.
I want one of those…for the sheer evilness of it!
I had this problem with an old cordless phone that had a flat battery.
My SO hid the damn thing after I thought that I had thrown it away.
Flat battery reminder beep at weird and wonderful intervals.
Solved when my son wandered in and said, " Where’s the flat battery?".
Well, I hate to be anticlimactic (an antithetical notion!), but he says the sound just… went away. He carries his cordless around in his pocket – it’s hard to picture him not returning it to the cradle at night, but I guess it’s possible.
He did mention that his electronic doorbell has been known to make sounds that are not the doorbell sound. I’m wondering if there’s any way that, say, someone’s garage-door-opener could interfere with the doorbell & cause a different melody?