yep, i had that problem the other day. it was bit hot for a couple of days here in perth so i was giving my A/C a hammering. (usually a non-a/c person)
anyways i started it up in the morning, after leaving over nite, obviuosly durrr... when the car jumped to life there was this weird noise. i had a listen for a bit, and it seemd to go a way as the car warmed up.
Could have been acat... hehehehe