That buzzing noise that you get when decelerating from 3rd to 2nd, and it goes away when you touch the gas. What's the deal with this. Is it normal for these cars. I own a 2000 S2000. Is it just one of those things or should I be concerned about aomething.

 

It's the springs in the clutch disc that vibrates in response to transmission backlash that's conveyed forward to the clutch. It's just a noise and has no affect on the car's function. If it really bothers you, you can do a clutch job. The updated clutch disc has little to no rattle, but there is no guaranty that it will completely go away.
This has been discussed repeatedly since the year 1999.