2010 S40 R-Design Down shifting sounds.
 

First of if I have asked this in the wrong place sorry. I am new.

I am currently on a long cross country back to the east coast from AZ. When stuck in traffic I was down shifting to slow down. going from 4 to 3 and 3 to 2 and 2 to 1 I could hear a weird sound like a wining sound. But when the revs match or as soon as they match the sounds goes away. I have not herd this sound before. I have looked around on line and thus far I have found nothing. The car has 75k miles on it. Any ideas on what it could be?