What is this noise?
 

3.2 Carrera making this horrible clattering rapping noise, but only under the following conditions:

• Engine warm
• 2800 - 3500 RPM
• Light throttle
• Light load

I cannot reproduce the noise with the car stationary. Open the hood and rev it up and down through the magic 2800-3500 range, no noise. Hold it at 3000 RPM, no noise.

Only if I am driving the car, accelerating gently, at that RPM range, do I get the noise. Wide open throttle? No noise. Off the gas? No noise.

This car has made this noise as long as I can remember (7 years, 15,000 miles). It made it before I adjusted the valves. It made it after I adjusted the valves. It doesn't smoke, doesn't burn oil, goes like stink, no symptoms except this noise.

I have considered the following possible noise sources, and discounted them for the reasons given. I am looking for other ideas (or for reasons why I shouldn't have discounted these possibilities).

• Valve train noise, broken stud, loose rocker shaft. These should all make noise at any RPM, with or without load.
• Detonation/pre-ignition. Noise is perfectly timed with the engine. One clack per exhaust pulse. Also, noise is happens regardless of ambient temperature, and only at very light throttle.
• Rattle from auxiliary PCV valve. I disconnected the control vacuum line without effect.
• Rod knock. See valve train noise. Also, unlikely to not get worse over 15K miles.
• Chain tensioner noise. See valve train noise.

Thoughts?