My 79 Van does the same thing. Tried 4 different distributors, 3 TQ's no change. In the van, you can change timing while driving, can make it worse be advancing timing, but even major retard the sound never goes away. As you say, if you back off over 2200 rpm, it stops. Also noticed if the engine gets below ~8" vacuum, noise stops. #7 plug looks a little white, but can't find a vacuum leak. Taking a couple days off and I'm going to get after it. After about a year, it is no worse, and hasn't fixed itself yet, or blown up.