98 Explorer, I know I have the dreaded heater blend door problem, but recently it just started to whistle from the vents as the rpms increase. It does it in all positions except when the **** is in th "OFF" position. I checked the vacuum lines to the heater control, from the glove box, but didn't see anything wrong. Any ideas? It's hard to pinpoint where it is coming from while driving. Does anybody have a good pictorial of the heater box with blend door?

 

It's possible that the blower motor is causing the whistle. To verify, unplug the power to the blower motor. Then try all settings. If it still whistles, then it would seen to be a vacuum leak.