1997 discovery runs good until the engine warms up

replace engine coolant temp sensor

this sounds like the engine oil cooler hose was leaking.

DO NOT run the engine again until you fill with oil.

start the engine and look for the hose that is leaking.

Piston knocking on the wall of the cylinder . The sound is smaller because as the engine (and especially the piston) is getting warmer it dilates and the gap gets smaller.
If it's that loud then it's time to repair the engine - if you're lucky you will need to fix just one cylinder. More likely you will have to completely overhaul the entire engine or buy a remanufactured one. Hell, if you don't have the money, even a good running one form a junkyard will do.

knocks when cold ( Start ), then sort of diminish or disappears when warming up ???

Knocking goes faster when accelerating when cold ???

If Yes, then Piston knocking on the wall of the cylinder . The sound disappears as the engine is getting warmer it dilates and the gap gets smaller.

If this just started,

1. Put quality oil & filter ( Mobile Full Synthetic Oil & Mobile Oil Filter or Purolator )

2. Do not put cleaning agents mixed with oil.

3. Personaly i've put "Restore" Engine Restorer & Lubricant in cars with more then 200 000 km and the results are amazing. But only "Restore". My Volvo is at close to 300 000 km, and it runs great. It's not a cleaning agent.

By experience if the knocking just started you can put an easy extra 100 000 km on the engine before a rebuilt. Quality Oil & let the engine warm up ( 3 mins ).

tune up, fuel filter, air filter, dirty throttle body, put good one to gas fuel injector cleaner.

Have you fitted the new exhaust?
Running it before may have knocked the timing out, which is causing it to missfire.

which engine do you have

Check firing order 1,3,5 in the back next to firewall 2,4,6 in front check coil packs. If o.k you could have burnt valves. As for the knocking If its a heavy kock and it happens all the time even at idle it could be the main bearings on the crankshaft.

big end or main bearing.

Thanks everyone for your interest and help for my problem.It was the power steering pump making the noise,got it fixed and all seems good.