I'm glad you asked. I was wondering about this too ... if something was wrong.
This is my experience:
I bought the mac mini 2009 in the first week it was released. It had a Pioneer drive and was exceptionally quiet. After I returned and got a new mac mini, due to a loose power connector, I found that the drive in the new mac mini was really noisy. I checked the specs of the drive and found that the make was different: OPTIARC, a division of Sony.
Hope this helps.