For those of you who were following along previously...

My funny noise was there again last Saturday when I was flying. So I took
the plane to my mechanic today to have them take a closer look.

Interestingly, it turned out that there were at least two possible
explanations, as there were two places one part of the plane was rubbing or
knocking against another part of the plane. One was a couple of spots where
the turbocharger blanket (the insulated heat shield around it) was rubbing
on the inside of the cowl. Another was where an aluminum part of the engine
baffle was too close to the cowl frame and was rubbing there.

To address the two issues, the blanket was reshaped (it's a soft aluminum
with some sort of insulating material underneath...probably asbestos, but I
never remember to ask) so that it didn't rub any more, and the engine baffle
was trimmed a bit to provide more clearance (the part needing trimming is
outside the airflow, and an insigificant amount of material was removed).

I did not notice any noise on my flight back. It's probably too soon to say
for sure that the noise has been fixed, but I think it's likely.

As it happens, while the cowl was off, a couple of other issues were noted,
one serious and one not. The not serious issue was that the engine mounts
are starting to sag a bit. The sagging is the actual reason the portion of
the baffle was too close to the cowl structure. They are still within
limits, but I will be looking to replace those next annual inspection.

The serious issue was one that probably would have gone undetected until the
next annual inspection, or until the part failed, whichever came first. It
turned out that the barrel nut attaching one end of the oil line that
connects the two oil coolers had cracked. Yikes! The crack had already
progressed about a quarter of the way down the nut. My mechanic asked me,
"do you believe in Providence?"

Of course, the cracked nut was replaced before I flew back. It almost
certainly had nothing to do with the noise I was hearing though.

Anyway, I'm glad that I stuck to my guns about looking into the noise. It
would have been so easy after having inspected the engine carefully to
ensure its health (at least where it might have been making noise) to just
keep flying the plane like that, with the assumption that the noise was
nothing serious. It turned out the noise probably WAS nothing serious, but
we managed to find something else important in the process.

I'm not necessarily superstitious, but sometimes it's good to just listen to
what your gut tells you. You may not understand why it's telling you
something, but it might be important nonetheless.

Pete