klinking noise front of motor

[youngblood72]

I have 1983 bayliner with a volvo pents aq 125 ans when I try to start it up it hesitates then turns over good then stops with some kind of clunking noise coming from the front of motor. I put a break bar on the front crank and it frees up then turns over and starts right up but I stop it right away whats going on with the motor.