OK, so in EQ it was DING!, as in "DING! level 20." In DOAC and AC, it was DING mixed with some NOT-DING etc.
Anyway, the sound that is actually made is more of a Whoosh than a ding in WoW.
So our guild was having the stop-saying-ding-this-is-not-EQ conversation and it was decided in our guild that we would instead say Boomshakalaka. "Boomshakalaka! Level 20"
It has caught on strong. We have a fairly large guild for how new the game is and the group is very active...so the boomshakalaka has become our levelling sound.
Maybe this should become the official levelling sound of WoW?