strange noise

I have 1995 Mazda B2300 rear wheel drive (stick shift). Recently I started hearing squeaking taping noise from rear axel. Noise is only comes then I am driveing in a gear, then it's neutral there is no noise. Car is runing nicely, but I hate that noise.
Maybe somebody have some ideas what could it be?
thanks

I have a 2000 B2500 series truck and I heard the same thing. My problems was the fire shield on the catalytic converter. Part of it was un welded. I only heard the noise at 3,125 RPM for whatever reason that was the frequency that it began vibrating at. I put the vehicle in park and slowly revved up the engine until I heard the noise and a friend was able to quickly locate were it was coming from. Try that and see if it helps you.