Loud ticking on newly rebuilt engine

I posted this on the Massey forum, but I thought I would run it by everybody.

1981 MF 285 with A4.318 Diesel
I just did an out of frame complete overhaul back at the house. Ran the engine off an on for a total of about 3 to 4 hours while fixing other items on the tractor (steering, hydraulics, etc). Re-torqued head, checked valve lash and decided everything was good to go.
I hauled it out to the farm to put it to work. Got it out there late and ran it around the fields under no load on a joyride. I went home that night and convinced myself I should change the oil before I put it to work. (it had Castrol GTX 10W-30 and Lucas Zinc additive). The Castrol was for gas engines, not Diesel)
I arrived at the farm and drained oil and put in NAPA 15W-40 and new filter.

I plowed two rows and a loud ticking started (a little more often that one a second). Simultaneously it started running a little rough. I shut it down (it didn't run more than 10 seconds after the ticking).
I pulled the valave cover and checked the valve rods, nothing bent. I didn't have a feeler guear to check the lash.

Anybody got any ideas what is going on? I am afraid to start it back up.

Take a look at the valve springs to see if one is broken. Hal

Check your manifold gaskets. An exaust leak will make a ticking sound, and an intake leak could cause a slight miss.
Loren, the Acg.

I had a d 15 that only ticked going up hill we finnally found it . the oil pump intake has a cotter key holding it in place and fell out.one trick we used is a short piece of garden hose one guy puts it to his ear and the other moves it around the engine sort of a scope like docs have . warning keep an eye on engine guy some have been noticed hooking up water and turning it on!!