Whats the loud grinding sound as I shift back thru gears R-P ?

I have a 1986 Monte Carlo with a 305 engine not sure about the transmission. When the car is started and I have the brakes pressed down and I shift through all the gears ( P-2 ) the car doesnt "jump" into gear. Then when I shift back through the gears ( 2-P ), as I shift through R-P there's a loud grinding sound for about 5-10 seconds or until I turn it off. I've searched numerous of sites for information and cant find it anywhere. SOMEBODY PLEASE HELP !!!