Which way do you turn the idle adjustment screw to raise the idle?
When my idle drops down to 750 rpm my blower starts to make a slight racket. When I take it out of drive and into neutral the revs to go 1000 rpm and the noise disappears.

 

When facing the engine bay, turn the screw counter-clockwise. You should hear a slight increase in idle speed.

 

Well if it is that high in neutral the ideler should be fine. The ecu will always override and set rpms at 650 when in drive.

 

That tells me there's somethiig not 100% with your blower. Your only masking it by increasing the idle. Try adjusting the belt and checking the mounts. What ya gonna do when the noise starts happening at 1000 RPM, turn things up to 1500?

 

I believe the blower is fine. In a thread to Iansw I told him I discovered that the drive belt would vibrate like a rubber band on the left side of the 3.33" pulley at low rpms. This in turn makes the blower give off a rattle. I've adjusted the belt tension so many times that I've got it at the point that I get no belt slippage and hardly any rattle. It's the wet and cold winter conditions that seem to plague me for the last 3 years.