I had to go back and reread this thread. I thought there was a comment that it could be heard, occasionally when free revving it, but as of now, it is only under some load. One way to see if the engine is hearing knock, or some other noise that might retard timing, drive the car in diagnostic mode and watch ED17, Knock counts. It is normal for some knock counts to register just from startup noise, so don't worry about the number that may appear there, this is normal. The thing to look for; is the number increasing during normal driving, specifically when you hear the sound, or the engine seems to struggle. Has the fuel pressure and delivery been verified? I know we headed off in the direction of ignition, but I believe you mechanic questioned fuel delivery way back in the beginning.