My Mini has had a horrible buzz/resonance for the past couple of years. The noise varied depending on road speed and rpm, but it was present most of the time and it seemed to come from different parts of the body depending on what section of the car resonated at any particular time at a various frequencies. I was convinced it was due to me fitting a new intake manifold and having the bottom of the HIF carb just barely touching the firewall. So I fitted a thinner carb spacer and also an adjustable engine steady to move the carb well away from the firewall. I figured that would do it, but after the car warmed up the buzz was still there. :-[
So this past weekend I put the Mini up on 4 jack stands and went on a hunt underneath with a flashlight and pry bar to see where the engine or exhaust was touching. It took me about 30 seconds to see that the bolt securing the passenger seatbelt stalk was too long and was touching the RC40 resonator . D'oh! I installed a shorter bolt and Bob's you uncle!
Yep that will do it! Probably a good thing to get the carb off the firewall anyways.