Highlander

My car is making this "clicking" sound - sometimes - when I get a little heavy on the gas. The sound is pretty much what I've always thought sticky lifters would sound like. It happened right when I got the car back in November, and after I realized the yo-yo's at the dealer had filled it with 87 octane, I started running 93 octane. That seemed to take care of it, but it still happens every so often.
I do run only 93 octane now. It doesn't seem to matter if the car is cold or warmed up, or even the temps outside. Ideas?