ratcheting noise in 5th gear deacceleration

Hello all, I have a 89 accord 5 speed 165k miles that is our "rainy day" car. It has been a great car since I bought it 3 years ago. I have noticed a problem developed in the last few days. In 5th gear(and only 5th) there is a clicking/ratcheting noise from the gearbox area. It only happens upon de-acceleration while in gear, push in the clutch, or just barely pressing the accelerator, it goes away. Car shifts flawless, and the noise only happens while coasting in 5th. All other gears, or turning, reverse, anything else it's perfect.
I take very nice care of this machine and this is the first problem it's had since I have had it. Any ideas?
Thanks, Michael