it could be your disturbutor and if it is, its probobly the plate under it. On Z's it's the large round plate you see directly beneath the rotor that holds all the "bits". The point is that it has a round plastic cage sandwiched into it which holds several little ball bearings, so it can rotate smoothly. this cage melts and crack quite often. You can test it to see.
the way to test yours is to take off the distributor cap, and while watching, suck hard on the vacuum hose leading to the dashpot on the side of the distributor. The plate should rotate clockwise smoothly about 1/2", and snap back when you release pressure. If you can hear your breath hissing from the distributor, the vacuum solenoid is probably leaking. If the plate crunches, sticks or moves poorly, the cage in it is probably bad.
You can take the plate out with the distributor in the car and replace it. Nissan dealers used to carry them for $45. Buying a new or rebuilt distributor today is probably the best solution. Good luck hope I helped.

 

It could be a lot of things , untill you find out you best keep your foot out of it, you may end up with a rod sticking out the side of the block..The engine is telling you somthing . You need to back off, I could be a rod but it sounds like a rest pin , wrong spelling, the part that connects the rod to the piston. If it is either one your days are numbered If y ou want to save the engine you better take it in and have a mechanic have a listen before somthing lets go.Just because the engine is still running strong tell me nothing other than it is most likely intune. If you drive it untill some thing lets go , you most likely will gunnysack the head as well as the block.

 

If you hear the sound coming from around the valve cover area, and the noise sounds like an intermittant rolling rattle that gets louder and more steady as the engine warms but not the familiar clicking clattering of the normal valvetrain activity, try to locate (within a 3 valve area) where you think your rattle is happening. Since you checked the valves for clearances after hearing it, I am thinking this is where you hear it the most clearly (guessing) Anyway loosen the three valves adjustment nuts on the valve guide dowels and pop your 3 rocker arms off (remember where they were because they have to go right back in the same spot they came from) and check the valve seats and the top of the valve guide dowel peen for wear. If one looks more worn than the others you will have to replace the valve guide dowel. You should also consider reworking the head since the rocker arm seat is likely to be worn also but you should not replace it unless reworking all of them as you could damage the cam.

 

Check the crank pulley for looseness too. Will need to loosen the belt to see it.