Noises problem
2003 Kia Spectra 4 cyl Two Wheel Drive Automatic 125000 miles

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 whinning 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 bearring in the a/c motor... I brought it to a mechanic but he couldn't find anything wrong and told me not to worry about it. I hope that it's 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