I also remember it starting somewhere around 2002, around the time of themes like "Party Zone" and "Doc Shock" and "Xtreme Show" where Tom was goofing on radio masculinity. I believe it may have started with a Petey call (that's the earliest I can remember him doing it). I remember it just being a kind of macho noise, like the audio equivalent of a chest bump, or a general "get pumped" kind of noise.
I'm nearing the end of 2003 in the archives (listening to one a day M-F) so this isn't terribly too far from my memory.