1998 355 F1 GTS Engine Clicking Sound

I don't know if what you call it is a rattle. It sounds more like a clicking to me. In any case, it is consistent with the RPMs, from low to high. It seems to come from the passenger side of the engine bay. I first noticed it at the beach this past summer. It went away after it warmed up a bit so I thought it may have been related to the temperature or humidity. I just drove about 250 miles and it was there pretty much most of the time. I say "most" because it did actually go away for some periods, hot or cold. The engine continues to run very smoothly. No issues with the transmission either. It had a major service in April 2008 at 10,000 miles and I've put on about 3,500 miles since then. The bonnets are tight, so I know it's not that. I've looked at some of the rattle threads on here, but I'm not sure we're talking about the same thing and many of those were for earlier years. My naive sense of it seems that it's rather harmless, but I'm curious to see if anyone else had something similar. Thanks

 

Thanks all. Concerning the headers, both have been replaced in the last two years and I changed the cats to Hyper-Flow to make it breathe better. This car hasn't been on the track either so, high (hot) RPMs have been pretty rare (save for the occasional unimpeded expressway onramp, of course). I haven't driven it since I wrote the last post, so I guess I'll report after the 12/19 when I take it out again in the cold NY winter.

Speaking of header cracks, though: Is the theory that the clicking sound is actually the sound of air escaping and not some kind of mechanical issue of components touching each other?