I have a 2001 Lariat SCrew. Today I noticed a noise coming from the AC compressor. After some troubleshooting, the noise is only there when the AC is NOT on. With the thermo set to regular vent, floor or off position I get this rattle/squeaking noise. With the AC engaged it sounds just fine.

Not sure if it matters but I have the manual HVAC controls.

Any thoughts?

 

Sounds like your AC clutch.

 

Time for a new Motorcraft clutch plate. Your old one has separated.