not sure about the noise change but my mate had a mk1 mondy and that did the same , never heard it on my mk2 , its probably the direction flap thing moving as for them only working on number 4 setting the resistors have burnt out , they are available from ford but they not cheap , better off going to a scrappers , the resistor block is under the dash on the passenger side / to the right of the footwell , its a little white block with a plug on it and is held in with 1 screw or sometimes riveted on

hope this helps