My 2wd cos has just developed a problem,

If I start it all I can hear are very loud tappets but after 5-10mins it goes. But intermittently it will come back and then go away??

I am guessing the pressure relief spring in the oil pump is sticking or has an issue but the pump is brand new and covered 1000 miles??

Any advice before I replace the pump. I have replaced the lifters and it is exactly the same.