So I just got back from the Santa Monica Subaru dealership. I had called them about this noise specifically and the *amazing* amount of A/C condensation that was dripping out of the bottom of my car.
Since the noise is easily re-producible, I hopped in the car and revved to about 3k RPM for a second, made sure the AC was on full blast, and slowly let the RPM drop. At about 2.5k RPM the chirp/tweet happened (it was really loud inside a shop with the hood up, too). Both technicians standing there smiled and nodded to each other.
As I turned the car off, they told me that this is a "normal" sound, and is the A/C clutch engaging. I told them how often it happens (every 15 seconds or so) and that it was annoying, and asked if there was anything I could do about it. They of course said there was nothing to be done.
This is in-line with all that has been posted, but I just wanted to add my $.02
I'm thinking about trying some sort of grease on the pulley wheel, but will need to look into it more.
The best part about this noise? My old car ('95 Civic EX) used to make the same exact noise. It drove me crazy enough to not run the A/C in that car (it barely worked anyway). I was excited to have a new car that didn't have such a silly little issue...and when I heard it the first time I knew exactly what it was and almost cried a little. 700 miles in and I had another chirping A/C!
(Oh, and the excessive amount of A/C condensation pouring out of the bottom of my car? Totally normal, too...apparently)