Engine Noise, please help

Hi guys, I have a 2008 si, it's got some engine noise. Can you guys identify what kind of noise it is?

Here's the characteristic.
More significant when cold
- even thought engine oil is warmed (read from the guage), I'll still have that noise
- If I drive hard for a minute or so, the noise is reduced substantially, or disappear

Only when engine is under load
- around 2000 rpm, when I try to accelerate

Sounds like something "ticking"
- ticking sound, but really fast paced.
- So fast that it almost sounds like a constant sound.


I put in 91 octane in the si (91octance is the highest there is in Calif)

If anyone knows what kind of sound/noise it is, and what's causing it, please reply and let me know.

thank you very much