My mini has been making this noise for quite some time, and its getting worse. Always on accelerating, here at around 3000 rpm in fifth gear. Sounds just like a bolt fell somewhere and now bouncing around, but idk. It is coming from the engine bay, left side of the vehicle… sound just like somewhere near drivers footwell..?! Please help..

 

I can't hear it in the video but based on your description of where it's coming from and the fact that it only happens in a certain gear at a certain RPM makes me think there's something internal in the transmission that's off.
Do you have the 5 sp or 6 sp trans?

 

Sorry.. I will try to make another, more demonstrating video. It doesn't happen just at the same RPMs, but almost anytime I am accelerating with WOT for more than 2 seconds.. the car otherwise drives fine. What you think could fail inside the transmission to just make some sound but otherwise work well?