Thank you for your reply I figured it out...It turned out to be were the plastic disk that sits on top of were you screw the truck into the truck wasn't screwed in all the way so it was causing the motor to be loose and have play so the spline on the motor wasn't catching the gear in the truck.

My Sd 90 heartiage unit makes the same noise on the rear truck like the gears free spin in the forward position but they don't in opposite postion so I don't know why it does that it grinds like crazy I tried using a different truck and it didn't work and the thing is is that the engine isn't a year old yet so I don't know why it would have problems so if u could tell me that would be great.