[Eric Wachsning]

Sounds like the idle on that engine is too low, they don't like to idle under 650 and when you put it in gear it will drop by another 100 rpm which will increase the chatter. It also could be the engine needs to be tuned up and / or timing checked. If the motor is not running perfectly it can cause backlash that you will hear in the trans. It could also be the damper plate is getting worn and will need to be replaced. This is not super expensive but the trans will have to be pulled to do this job and then put back. In summary try raising the idle to around 700-750 when in neutral and see if the noise goes away when putting it in gear. Considering you have not bought the boat yet this would cost nothing to check. If this does not cure the chatter then I would have the seller check the timing on his nickel. Hope this helps. I had the same issue on my 31