nasty noise when i shift into reverse

i have a 92 4 door teg and sometime when i put it into reverse it makes a horrible grinding noise . just wondering if anyone has an idea of what it could be.
Note: the car is automatic and has an after market shift knob on it also the fluid is fine:rofl: :rocket:

The reason this happens is because revers dose not have syncros in order for this not to happen put ur car in 1st gear before you put it into reverse this wil line up the gears and not cause any grinding… in case you are wondeirng syncros line up ur gears when u shift thats why u can shift from 1st to 2nd withough waiting for the motor to wind down or somthing but in revers they do not line up unless they are lined up becuase you ust took it out of a gear.

EDIT: i jsut noticed you said your car is auto …and what i described was for a 5spd i dont know if it still applies