A/C compressor rattle but only in Drive at idle...

[derekjl]

As the title says, I am getting a faint rattling noise from my A/C compressor but only under specific circumstances. It only happens when the truck is in Drive and sitting idle, so this would be at the lowest possible RPM. When I hit the gas or even when it is in Park, no rattle. I know it is from the A/C compressor because as soon as I turn the A/C button off the noise stops, fan running or not. Any ideas? Might sound weird, but is there any way to increase the idle RPM of the truck when it is in Drive? Looks like it hovers right around 600 RPM.