Here is a good tip about diagnosing engine knocks from Andy -*.||||.*-
"Get a timing light and hook it up compare the knock to the timing light -- if the knock happens one for each flash of the light your noise is in the top end and you can rebuild the top end of the engine, if the knock is twice the speed of the timing light it's in the bottom end and you should do a full rebuild."
Great for air tools, filling tires before getting back on the road, and air lockers. Check out these web articles:
http://www.jedi.com/obiwan/jeep/onboard-air.html
http://www.europa.com/~bateman/jeep/78cj5/mods/air/
http://www.huv.com/jon/jeep/
The best place is in the exact center of your metal roof.