I thought that "flush" involved adding some sort of solvent, like Berryman's, to the oil before you changed it. I've also heard it as putting that extra quart down to get some of the leftover junk out. Personally, I'd do both. My car is doing the same thing as yours, although I don't hear the noise below 2k RPM. Running the Berryman's combustion system cleaner (part # 2610) reduced the noise. I've done mostly highway driving and doubt that it's caused solely by carbon buildup.
