Dealer fixed air conditioning after 65000 miles, now a high pitched hum.

What is your question?

Now there is a high pitched hum when it hits 55 - 60 mph. It only lasts a second or two, but it accures about ever minute or so. When you turn off the air, it goes away. Any ideas? The car is great otherwise. One owner who took great care of it. This is my first Miata, but it's always been my "dream car". It 's a convertable if that helps.