For a change, I can shed some light, and high mileage experience as well. As of yesterday I now have 2 300m's, a 2000 and a 2004 Special.

My 2000 is about 90 miles away from hitting 300,000 miles. I also had the tapping noise that you describe at around 140,000 miles. I thought it was an exhaust leak, so I kept driving it until I had time to deal with it. During that time, I never babied the car and ran up close to another 20K on it.

It turns out that I had 3 lobes wiped off one of the cams! This was with synthetic oil since I bought it a 33K. I had a good, then unempolyed Mopar mechanic of mine discover this. He ended up replacing the cam (pass. side I think) and all of the combo rockers on that side. The head needed to be pulled to accomplish this.

That cured it. The car still runs well (taps somewhat now on startup in the cold) but only uses a quart of Mobil 1 10W30 in about 5000 miles. My only other advice if you do this work would be to use a MOPAR HEAD GASKET.

Good luck!