4.0l Vibration 900rpm cold start

Just recently I've been experiencing engine vibrations every morning. Truck starts fine, revs to its normal 1400-1500 rpm, smooth as butter. Through it in drive, drops to the 600 rpm range, smooth as butter.
But for maybe the first minute or two there comes some strange resonance at 900 rpm.
Only happens on cold starts, not sure if its anything to worry about?

 

Hey, any update on this? I think I'm having a similar issue, sounds to me like it is coming from the ac compressor.

 

Haven't heard anything in a while. But I have developed a rattle from an exhaust component. Possibly heat shield or the cat.

 

Search YouTube for how to recalibrate your idle and also recalibrate your gas pedal. It works, only takes a few seconds.