UPDATE:
Switching off the main breaker was the first suggestion taken. It did nothing to minimize the buzz, in fact it was more noticeable. (Oh and Steve, you've been in my basement/bar room - they all have sump pumps.)
Did the ear-to-ground thing outside. Could not hear anything like it.
Although I do suffer from mild tinnitus (been shooting and listening to rock & roll all my life), I know the distinct difference between "ringing ears" and an audible 150 Hz buzz.
The phone tone story is funny because that same thing happened to me in an evening class one time and nobody could figure WTF it was. Ends up it was my phone, and I didn't hear it.
So finally, I called my g/f over for dinner (she brings her young daughter). She has unbelievable hearing abilities, she hears nothing - just gives me a funny look like I need more sex.
Little brother and his two kids come over and I hear no noises except a houseful of laughter, TV and puppies playing.
Now the house is quite. Went into my walk-in closet where I heard it loudest and it's gone.
... Summary in order or probability:
(1) Inclined to think it had something to do with the nearby construction.
(2) Sex deprivation.
(3) Senility setting in.
(4) Bad-ass motherf*cking termites.
(5) They really are out to get me. (sleeping with tin-foil on if it comes back)