Not sure bout the MDD, but all orginal fans in my QSs had a small temp-sensitive resistor mounted directly at the fan.
My advice (as little helpfull it may seem):
-get yourself some electronic fan-control preferably with temp-displays
-mount the the temp sensors with the old fans, see how hot it get where
-replace fans with ones that have average or above airflow
-regulate them down to the point where temps are as good as before
And for finding out where the noise comes from:
Stick a screwdriver in each fan after another and listen to the difference (just not for long...)
