M42 engines are known for the "sewing machine" sound after 150K. I suggest taking the valve cover off and doing a valve adjustment.
its not that hard, just need a feeler gauge, socket wrench and an open box wrench.
I had a really bad ticking problem with my M20, the valve adjustment made it near silent.
BTW, if your problem gets worse, chuck the motor and get an M20 or M50. cheap motors and easy to swap.