Well, as promised, here's my follow-up...

Went down to TIW today, after picking up Ron my mechanic first. Yesterday,
one of his employees went over to check the airplane out, full-power run-up,
inspection, etc. He found no sign of anything wrong.

I did my usual thorough preflight, while Ron double-checked some stuff. We
did find a loose rivet on the fixed trim tab on the elevator, but nothing
that would make a noise you'd be able to hear in the cabin.

With nothing left to do but fly, we taxied out, with the plan that we'd
climb over the airport and then depart to the east (back to my mechanic's
airport) if everything looked good. Well, we got to the runway end, where I
did my run-up, and the right mag was *really* rough.

Ron was pretty sure that we were just looking at a fouled plug(s), so we
pushed the power up to burn the crud off. It took a while, and pretty high
power settings, but we actually managed to clear the plug(s).

Oddly enough, his employee found no plug fouling the yesterday. This was
apparently some new accumulation.

Anyway, once we got a normal runup, with both mags running nice and smooth,
we were ready to go. Took off, did our climb (and discovered that TIW has
one of those pesky Letters of Agreement with Seattle Approach where they
abdicate ownership of their airspace above 1700' AGL), and everything
sounded and felt fine, so we headed back to his airport.

The whole flight went just dandy. Ron's current theory is that there was
some small chunk of carbon floating around in the cylinder, that
intermittently was affecting the plug. He says he's seen that before, and
generally the debris makes its way out of the cylinder through the exhaust
before too long. If it got really stuck, it would explain the rough mag
we'd found on the run-up as well.

Without anything to fix and the airplane apparently running fine, I took it
back home to PAE.

Was Ron right about my problem? I have no idea. I'm a bit skeptical, just
because the problem didn't seem like the sort of thing that was affecting
just one plug on one cylinder, but it's hard to say. I one time lost an
entire mag shortly after takeoff, and the only sign that there was a problem
was that my fuel burn was a little higher for the same power setting. The
engine ran nice and smoothly, and I didn't even find out the mag wasn't
working until I did my run-up for my next flight. But I suppose in that
case, all of the cylinders were being affected identically and that if just
one or two was being affected, that might show up as some roughness.

On my way back to PAE, I was reminded about another effect that I'd
forgotten about. Due to the way the sound waves propagate within the cabin,
there are "hot spots" of noise. If I move my head to a particular spot, a
noise that is otherwise inaudible becomes apparent. I was unable to find
such a spot at which I heard a noise identical to what I heard last
Saturday, but I didn't remember this until I was in cruise flight. If I
remember, I'll try moving my head around during my next takeoff and see if I
can come up with the same noise I heard before. Maybe it WAS just the
result of a particular kind of resonance after all, as Larry suggested.

Anyway, that's all I know for now. I realize this thread belongs as much in
r.a.owning as it does here, but I haven't been following that newsgroup, and
it seems on-topic here too, so here it is. Thanks for those who showed
interest...I hope it wasn't too much of a letdown to not find out exactly
what the problem was after all. I guess a theory is better than nothing.


Pete