Fortunately I drifted off again and got a full night's sleep. But I was really curious, and asked Jean if she had heard it anytime during the night. She says no, but will often sleep through things that wake me up, despite claims that she is a light sleeper.
So this morning I followed my hunches, and found this link, with the key phrase,
In general, work hours will be Monday through Friday nights, 8 p.m. to 5 a.m.
There is also a section on "nighttime noise", as well as a notice that they're building a new stretch of "soundwall", starting around SW Ponca Ct., which is just South of Umatilla, where we live. Ironically, it may be preparations for this sound-deadening wall that I was hearing last night.
Supposedly this has been going on for months, and only gets suspended in the winter, so I don't know why I didn't hear it before. But I feel this is the source of my mystery sounds.