Engine rattling?

I need some help troubleshooting
I just got a 1990 b2200 with a rebuilt motor, basically everything is new except the pistons. The guy I bought it off of said it rattles a bit when its cold because the lifters are non-adjustable but it goes away once its warm. Hhe did the work himself and sounded knowledgable. It rattled slightly when I test drove it, but it seemed to go away (I was on slow side streets and wassn't really stepping on it). I drove it home and it seemed ok, but had other rattles (body, exhaust) so I could have missed it.

I've had it parked for 3 or 4 weeks but finally sold my car yesterday and started driving the truck. Its loud, like a diesel when its cold, then the rattling gets quieter unless I step on it at all and its under load then its definitely noticeable. I have a tach and the rattling doesn't seem to affect the rpm at all.
Any ideas?

Could be spark knock. Have the timing checked could be off a little.

where is the sound coming from?

checked the timing, its right on, oil is full and clean. I think it might be the lifters, but the motor is freshly rebuilt including new lifters

check your fan bolts if its loose,tighten it and see if its gone.

its ur rockers. the have hydraulic rockers. there is little holes in ur rocker arms that the oil flows threw to keep ur tolerances tight. when these holes get clogged with gunk the oil cant pressurize the rocker so it just kinda bounces around. just remove the rocker assembly and clean it with carb cleaner and a lil brush. it will fix ur prob

*edit* these motors are overhead cam and dont have lifters.

or its spark knock which means a slow slow slow death to the engine....just means that the piston (which you say WASNT replaced with rebuild) is rattling/moving around....you can run higher octane fuel and it will more or less disappear for a time....but eventually...its guna need the pistons replaced.....If you run higher octane and it dispappears first off can you please let us know so that we know we have helped solve your problem for the time being, and secondly...we will know we have nailed the problem down and can work on a further solution from there.

Thanks man and hope we have helped solve your issue.

if you still cant pin point the location of the noise, get a broom stick or something of that sort and set one end on what ever you think is making the noise and then put your ear up to the other end. my grampy told me this and he's a retired british racing mechanic. it works like a doctors stethoscope. maybe it will help.