Quick hits:
-Profile gasket at 65k. A bit inopportune, since I had replaced the tensioner myself a short while earlier, but BMW did pick up half the tab. Also had them put in a new water pump at the same time, just for convenience.
-I should also point out that when I shot the clips, I used a long screwdriver to try to act as a scope against the valve cover, and didn't hear anything abnormal.
-The sound usually occurs after the engine is warmed up. Keeping a sustained rev in the narrow range usually makes a sustained noise. Passing through, it sometimes does, sometimes doesn't.
I'm relieved to hear that it doesn't appear that the engine is ready to grenade...yet. I've always thought the evidence leaned toward something external; I just have to make the time to poke and prod everything while someone or something can hold the throttle in one position.
Thanks for all the input.
