Yup, thats exactly what I have on cool down. A dripping sound from the top of the driver's valve cover, towards the driver's seat. Of course its probably not a drip, but thats the best way to describe it. Unless more people chime in saying they get this sound too but do not get ticking during driving, then I'd suspect you and I could be having identical problems.

This cool-down tick, combined with the fact that the driving tick gets worse with heat, made me think its a dead cat converter. You can find some posts on here with people who had a dead cat, as well as on the benz forums. A dead cat converter will heat up and the damaged area will melt and starting banging around again. The converter, since its clogged, will get absurdly hot, and the manifold in front of the converter will get much hotter too because of the clogged converter. All of this hotness combines for a loud tick on cool-down, and the damaged cat causes a hot tick during driving.

It all makes sense, but I could never isolate the sound. Also, the tick is so perfectly synchronous that a piece of cat clanging around didn't make complete sense.

I also notice my driver's side cat has a small bump in it. Maybe its normal, but maybe it got so hot at that small point that it slightly melted?

Does your car backfire when you rev it in park? If I rev it slightly, to maybe 3000rpms, I get pops when I left off the pedal. I'm just curious, I'm not saying its pointing to anything... Yet.

I wonder if my heat shield bending simply helped muffle a clanging cat?

If you stethescope the running hot tick, you'll hear it loudest at the cylinder closest the driver's seat. This again can point to a cat problem since thats the closest cylinder to the cat.

Two more things to point to a dead cat: An infrared shot of the area around the O2 sensor shows like 300 degrees celsius. Now thats hot, and its not even at the cat yet. You still have 5 inches to go before the cat. The other side's cat and surrounding area doesn't nearly show those temps. And also, you can't put your hand behind the engine after it gets hot because the air alone could burn you. Its that hot back there. Seems strange to get that hot.

I'm considering having some high-flow cats installed to help eliminate this as the problem.

I have no leak markings around the manifold->head area, and I don't seem to have cracks on the manifold anywhere.

I've replaced crankshaft sensor and plug wires to eliminate electrical arch possibilites in that area.

I have my stock pulley, and it definately has some 'play' in it that could cause ticking. I'm putting on a code3 pulley once it arrives so I think that, a new belt, and new s/c idler pulley, will help eliminate the belt drive system all together.

Throw any more findings you have at me, maybe we can combine for some real heroics here.