Welcome to the group, and congrats on your MINI! :wavey:
A couple of things:
1. A vibration / rattle at certain RPMs usually - to me - means you have something that 'resonates' at a certain frequency. Could be a loose plastic snap connector, could be something in your doors, or glovebox, or elsewhere, that rattles only when you hit that certain frequency - which happens to occur when you rev the motor Can you duplicate the noise sitting still, revving the motor? Is it only while you're rolling?
I know for sure that there's 'something' in my er... spare tire well area that rattles when I'm moving. I know for sure that nothing rattled when I took possession of my Clubman, and I've moved/removed enough to eliminate most areas. But I've heard of EZPass transponders being the source of vibration / noise as well.
2. Our cars are made in Europe, so I suspect they follow the United Nations Economic Commission for Europe (UNECE) Regulations. Basically, they allow for certain tolerances to account for various wheel sizes / diameters, and to ensure that a speedometer error doesn't account for speeding. In the US, there's an allowance of 5% (the indicated speed can be more by 5% than actual speed). MINIs aren't the only ones with this issue. But it's not a bug - it's a feature
Wish I could help you more. Good luck finding the source of the rattle! :itsme: