A/C whining noise when decelerating

Hi, my car recently started making an odd whining/humming noise coming from engine area. It only happens when slowing down and when the A/C is on. As the engine slows down, this whining winds down as well. My brother lives in another state but when I explained it to him over the phone, he said it sounded like the bearing in the A/C motor. I brought it to a mechanic but he could not find anything wrong and told me not to worry about it. I hope that it is nothing to worry about, but would hate to find out later I could have preemptively saved myself hundreds of dollars spent on A/C repairs. Any ideas? Thank you so much in advance.
Cheryl