Anyone else having a rough idle?

I have about 1100 miles on my STi now. It has been very hot and humid here lately.. 90-100 degrees farenheit. When I have the car parked and just sitting there letting the engine idle for a while.. the engine will rumble randomly.. Could this be detonation? I never notice a change in rpm when at idle.. it always stays at 500 rpm even if it rumbles (shakes the car slightly) or if I have the air conditioning on or off.

I am using 93 octane pump gas.. I notice this happening whether or not I've used a bottle of octane booster or not.

I have not noticed detonation happening under hard acceleration yet.. although I've yet to go past 6000 rpm.. and even then only during a slow pull at high speed in 5th or 6th gear.