Hear a rattling noise at 1,500 RPM

What is your question?

The car has 97,000 miles and has been well maintained. I made a trip of about 220 miles this weekend and half way through the trip I now hear a rattling noise at 1,500 rpm as it goes from gear to gear as I speed up. At about 1,800 RPM it stops making any rattling noise. Any thoughts what might be causing this? I drove half of this trip and stopped for about 3 hours and when I continued on it started making that noise. Car seems to run fine but makes the noise. If I am sitting still without the transmission engaged it does not make that noise but the trans seems to be shifting ok.