I had the same issue with my Rebel when it was less than a year old. The first winter I had a chirp/whine audible at idle. I quickly swapped the belt and felt all the idlers. The idlers felt fine and the belt didn't resolve anything. Scheduled some time at the dealer and when I got there, the noise went aware. Go figure.
The noise also went away for a couple of days after that dealer visit. They said no belt dressing was used, so I was puzzled. Then it came back when we had another cold spell of weather. It was a warm and raining that day at the dealer. I never rescheduled and it would come and go depending on the temperature outside. Cold weather it whined/chirped. Warmer weather it was gone. I suspected a slight pulley misalignment that was apparent with cold weather.
I never found out what it was since it only chirped in cold winter weather and I traded the truck that following summer. I had issues with the air ride and dealing with that nitrogen setup. I had no interest in keeping it when the air ride problems started coming up.