Phantom PURR sound

Once in a while and without an apparent reason for that, I hear my iMac playing the PURR sound. It happens once or maybe twice, but not at any defined timetable. It happens mostly when it's between XX.40 and XX.50 o' clock, but it's not a rule and it doesn't happen every hour. The only applications always running when I hear it are Microsoft Messenger and Skype.

Anyone with a clue for me? =(

THX!