Slight rattle at 2500 rpm

What is your question?

When my engine is accelerating at about 2500 rpm, I can hear a slight rattling coming from the engine. I can't place where its coming from other than the front half of the car because it won't do it in neutral, only when the car is in gear and moving. It also only happens if I give it some gas. If I really lay into the pedal, it goes away. I was told it might be spark knock, and that I should use higher octane gas, but that had no effect. I am thinking that it may be the transmission or the torque converter. Thoughts?