This will be a long post - I'll put the problem up front and will try to answer background questions below. Help would be much appreciated!

I have a 2011 Countryman S with 145Kms (90K miles) that is new to me. It has a new clutch (by mini dealer) a month ago as I bought it with a broken clutch return fork. Since I got it, it makes a weird whirring noise that i can't ID - I took a video as it's tough to describe. I've plunked it in YouTube, which isn't (apparently) compatible, so hoping this link is ok, but if not maybe I'll record a sound file: https://youtu.be/N2qdUAoepxw

The noise happens in all gears under light throttle, immediately gone as soon as you lift off the throttle. No noise with the clutch pedal depressed, can't reproduce by revving in neutral, it only happens while in gear and moving. I lifted the front wheels on Jack stands to see if I could find where the sound comes from, but can't repeat the sound that way somehow. What I've done:
- new clutch
- new serpentine belt (needed anyway; no change)
- drained and replaced trans fluid (no change - and happily no shavings in the fluid)

I'm not new to wrenching, or even to BMW's, but I'm about a month into my first Mini, and still getting used to the logic of how they go together, so help from someone who has more experience with these would be great.

I bought my 2011 countryman S with a broken clutch return fork because it's in great condition and the owner had service records, and because I talked to his mechanic (a mini specialist) before I bought. It has 145,000kms (90k miles), no modifications and no damage. The owner says this sound wasn't there before I had the clutch job done (I had the mini dealer do it because I wanted it on the road), but I'm keeping in mind that he might be somewhat mistaken where the truth is concerned. Not sure, he seemed pretty forthcoming so might be true.