And the noise continues...1997 JEEP Wrangler

At the end of June we posted a question about a tapping noise that our 1997 Jeep Wrangler makes when the engine gets hot. My wife heard the noise for the first time last week when she was sitting in traffic waiting to come into our new motorsports park here in Millville NJ. She was in the line for over 30 minutes and at one point she heard a noise that she thought was coming from another car. There were 3 lines of traffic coming into the park. As she got to the entrance point she knew it was definitely coming from the Jeep since there were no other cars next to her. It was a loud metallic tapping noise. When she put it in neutral, the noise got quieter, and it stopped when she put in park. When she started it about an hour later to drive it home there was no noise and the noise did not occur on the 15 minute drive home. So the mystery continues, and to this day, no mechanic has been able to get the car to make the noise. Any ideas will be most appreciated. Thanks, everybody.