I drive a manual R56 (MCS) with 66k miles. I sometimes noticed a chattering or rattling sound when I go up an incline. If I downshift the extra torque gets rid of the sound.

A similar instance produced the same sound. I was cruising on the freeway in 6th gear. I had to overtake a car to switch lanes. Instead of downshifting I decided to go WOT (pedal to floor), which reproduced the noise.

So it seems like I'm pushing the engine too hard in the wrong gear, but I'm concerned whether it should be reacting this way, and where the noise is coming from. Wouldn't it instead just accelerate really slow due to low torque?

Don't know if this helps, but my last visit to the dealer said there was a cylinder misfire, and attributed it to requiring a walnut blast treatment.

 

Sounds like detonation/engine knock. The walnut blast will help.