Hm. I know on cars with the hydraulic lifters, they can clatter a whole lot if they've been sitting a while. My mom's car sat for a couple months with a rusted/open fuel line, and its next startup it took 30 seconds or more for the lifters to 'pump up'. Haven't seen that behavior after the fuel line repair (it gets run every other / third week now).
Disclaimer: I don't own a M1008 and have some minimal direct experience with a friend's GM diesel 6.2. I'm hoping another forum member may help chime in.
Advice starting with the cheapest /easiest and moving up:
I'd put cheap new oil of the thinnest allowable weight in it and a new oil filter. Cut open the old filter and look for bad stuff in it if you can, preferably using tin snips or an oil filter cutter. If there isn't a lot of metal in the filter, then hopefully the engine hasn't previously been 'making metal'.
If you want to verify the lifters, pull the valve covers and glow plugs, then check to see if the valve rockers have play in them, rotating the engine by wrench. If all of them have play, then I'd suspect 'drained' hydraulic lifters, especially if someone with more experience can confirm hydraulics are installed.
If things look bad, pull the pan and look deeper.... but if not:
I'd check the oil pressure with a good mechanical gauge ($40 or less) at idle, and I'd run the engine only at idle for 45-60 seconds to see if the lifters start to pump up. If the engine shows signs of pumping up, then I'd drain the oil into a nice clean pan and do the 4bogginchevy's trick of 50% transmission fluid (if the pan's clean enough, just re-use the recently-used oil for this). I've had good luck with 1/2 qt trans fluid in my car for 2 minutes prior to an oil change, seems to help clean it out. You can also hold a dollar bill or piece of paper across the tailpipe to see if there is a cylinder that has a leaking valve, I'm thinking if a valve was sticking you'd be getting much more noise-of-doom.
Also the 6.2 non-turbo diesels tend to knock a bit when cold, especially if a couple cylinders are not getting good glow plug response. If there is a good bit of white puffs along with the knocking, I think that's a sign of a mostly-healthy engine. Checking glow plug condition when they are out might be a good starting point before pulling the valve covers. They swell and lose electrical resistance when they go bad.
Once you've done the 'flush' then put in good oil and a new filter for any actual driving around, assuming your new find comes happily back to life. The cheap oil would give you 2 flushes with 50% trans fluid if you want to re-use the cheap oil.
I'd also recommend at least checking all the other fluids before any drives. Best of luck, congratulations on the new ride!
