1997 Lexus LS 400 Scraping/Rattling Noise Under Car

Lately when I start my Lexus I hear what sounds like a rattling/scraping type noise coming from under my car. Thinking it was a loose tailpipe or muffler I took it to a repairman this morning and asked him if he could find the source of the noise. He checked the undercarriage thoroughly and couldn't find anything loose. He said it could be the catalytic converter (it's never been replaced), but not to worry about it. I'm concerned about simply doing nothing and the noise is very aggravating and embarrassing when I pick up my buddies for golf (they already kid me about continuing to drive a 13-year-old vehicle). Interestingly, I don't hear the noise once I'm underway. It's only when I crank up and move the first twenty-five yards or so. Other than the noise the car has given me no problems (never has). I just want to be sure that allowing the noise to continue without really knowing its cause could be asking for trouble.