A buzzing from the transmission when decelerating from 3,000 to 4,000 RPM can be caused by normal gear backlash noise in the transmission. Honda has an updated clutch disc to help reduce the noise.Google+
Car Problem Reports
Honda S2000 Buzzing from transmission on deceleration due to normal gear backlash
Honda S2000 Problem
Visitor, 2001 Honda S2000, 70,000 mi
Just bought the car, I never took the car to be inspected before purchasing it. I realized this buzzing, humming vibration noise first day after I bought the car. Took it to many places to be looked at and everyone had a different story. Its been a month and I still haven't found anyone I trust or believe knows from experience what the heck this issue is. Just from my research and reading great articles like this I would imagine its one of three things, transaxle, clutch, or transmission. Anyway annoying noise hope its not going to do more damage and hope I can find someone reputable that knows what there doing.