Knocking at startup, turns off after 10 sec, please help!

When head gasket leak has been going on for some time (or with a new acquisition, with anything over 150K) you want to check the Top Dead Center position of the pistons very carefully.

Acquired this (0-150 mm) micrometer depth gauge set, when we were clearing out father-in-law's house; it might even have enough reach to work through the spark plug holes. (attached)