Disclaimer: I'm not a professional mechanic, just some dude who has worked on his fair share of personal vehicles.
Did you run though some RPM levels then let off, while it was parked, to see if it makes the sound?
Maybe take it to 3000 RPM for 10 second, then let off back down to idle. Then try 4000 RPM or whatever the max is on a normal drive (no need to go higher than that, probably). Ideally, if your wife can handle holding it at a certain RPM, have her do it, while you stand outside with the hood open. If your hearing is shit, and hers is good, maybe she stands outside (next to the engine).
If you get nothing with that, then that would make me think suspension or drivetrain. How old is the vehicle? Have you been doing the recommended mileage maintenance? Maybe the rear differential is low on fluid or needs a fluid change, if the car has more than 100k miles on it. Same thing with transmission, maybe it needs a flush. Although, I didn't think tranny whine really went away ...or maybe it does: google has a bunch of questions on it:
Google search