Greetings and Welcome to Just answer.
Was it rattling at times before?
If so then I suspect the oi pressure relief or pump is the issue. The are common and are in the same assembly. This is around the front crank under the front cover.
IT could be that, or the o-ring for the pick up tube into the oil pump. those o-rings were common for that exact issue.
It sucks air so the pressure is still up, but air in the system will not allow the lifters to work properly.
Sorry for the delay.
I would not suspect contaminated lifters.
No, I would do the oring first. I ahve done a handful and that was the issue is 90% of them.