What was the outcome?
I have the same issue.
I know in the past on a Cooper, it was the shock tops.

Been driving with it for I don't know how long lol
Never been resolved and it's gotten significantly worse
It's funny no one can find where the sound is coming from
I've been to 3 different shops and they all have different opinions....
I think it's my rear struts but what do I know....

Any updates? Lol

I know this is a reach, but check your latch on th rear hatch.

I had a similar sounding noise and thought it's the rear wing bolts, it's something loose in the back, or a loose fitting of some sort. Nope. I went to the car wash one day noticed when the service tech pressed down on the hatch a very similar noise occurred. I went home and checked the latch. Nothing loose or out of place. I decided to try something different. I have a can of garage door spray lube handy. This is the stuff you spray on your garage door rollers, chain drive, screw drive, hinges etc. I sprayed this on the latch and the upper piece on the hatch.

So far the noise tends to go away for several months. i recently noticed it returned so I sprayed it again. It is a cheap fix. Might not work for everyone, but the hatch does tend to rattle a bit over time.

If nothing else, at least you don't need to get under the car to try this one. If it doesn't help, at least the hatch latching assembly will work smoothly.

Just my 2 cents. Hope this helps some of you.