Markiempje

sometimes interference happens when metal parts tough eauch other. dontask mywhy but it just is. try touching a metal part onyour chassis with a metal screwdriver your holding in your hand.
the same thing could behappening with your fan continiously touching your motor casued by vibrations fromyour motor or sudden changes you make with your car, like the ones you stated... you could try mounting it more Firmly to your motor to be sure its fixed. i know most cooling ribs fit perfectly onto your motor but with a slightly damaged motorsticker it could be just be the weak link...
for the same reason I've seen people putting their lead balance wieghts in shrinkwrap before they mount it with double sided tape, actually strange cause the tape isolates both from toughing yet they still do.

try mouning your ribs firmly with glastape

Mark