I have a 2005 R54 Mini Cooper S with the JCW package. It has an aero kit on it too. When I am driving and I touch the gas I hear a thunk thunk noise from the back passenger side. It's quiet enough I didn't notice it at first, but now that I know the noise is there it's driving me nuts. Anyone have any ideas?

 

Could be a number of things. Bad bushings in the rear suspension, loose shock mount, bad bushings in the rear sway bar, or even something as simple as a loose item in the battery box or rear side compartments. Start with those & then narrow it down. If your car is still under warranty the dealer will check it out for you.

 

It's your upper engine mount which is located on the passenger side. See if the mount leaked - you'll notice some dried up black oil around the area...

This should be covered under warranty. If not the part is about $70 + you need an E14 socket to get the bolt from underneath that holds it in place.