Bought a new long 2.8 Diesel motor new head pistons injectors and oil pump all new or machined internal parts and bench tested OK my pump . I have had what seams to be tappet noise , very loud at cold and noisy when warm and between 2.500 revs and up under load. Diesel re-builder/supplier tells me it's the pump and go and get a new one... appreciate any suggestions please Peter

Attach a mechanical oil pressure gauge and check the pressure. I would only use a full-synthetic diesel HDEO, with the proper viscosity, in your engine. Oil pressure builds 7 times faster with synthetics than conventional oil. The advantages of synthetics in diesels are well-documented. If you have hydraulic lifters in your engine, which I'm not familiar with, the synthetic oil will clean them and quiet them down.

If I remember correctly that is a flat tappet lifter engine. The design is now somewhat outdated, and I believe that Jeep was the last to use it in 06. It's the design of the lifter. I have a 03 jeep that does the same thing, and is in perfect operating condition. The synthetic oil does help, but it's not a miracle fix. It's the way the lifter meets the cam lobe, and the oiling system for it. I would not worry about it to any great extent. Mine drives me nuts in the winter monthes, but I just deal with it!
I do recommend checking the oil pressure to be sure you are in good operating condition, and a quality oil, as TX said.