take a guess at odd engine noise?

Hi folks.

I know this is almost impossible to diagnosis, but I have a ticking/tapping noise at idle that I'd be interested in hearing guesses about. It's a 340, rebuilt 15 years ago. Only has about 4000 miles on it.

I don't think the noise is new (didn't just suddenly occur), but may be getting worse over time. It seems to be emanating from the right side of the engine, but cannot say for sure. Maybe every other revolution. At warm idle (7-800 RPM), it's quite noticeable. At 1000 RPM it's less noticeable, and by 1200 RPM I can't hear it at all.

Any ideas? Thanks.