Heater fan and foot vents.

Firstly, my heater fan works reasonably as you would expect from a car with a diesel engine.

Although once you switch it to position 4 (iirc) it gets considerably noisier and much more vibration. (to a point where I do not use it this high) Any suggestions or reasonably simple solutions, things to look at.

Also, if I block up the passenger foot-well vents, would this give more output to the other vents, or do the flaps / assembly not work this way.

Cheers.