Listen to it with a stethoscope to see if you can determine where the noise is coming from. Sometimes you can get an idea, and sometimes you can't, but it's always worth a try.

Next you can pull the valve cover and check the lifters. They will all be a bit loose with the engine stopped, but you want to see if any are much looser than the others. You will have to turn the engine over a few times, as you can only see what's going on if the valve is closed.

If you think any are loose, you can remove the lifters, carefully keeping them sorted so that you do not put them back in the wrong place. Immerse each lifter in oil and pump on the piston with a tool. They should become very stiff after just a few strokes, and if not, all of them should be replaced.

I don't know if you have roller lifters on that engine, but if not, it is imperative to never run a used lifter on a different cam lobe than before. If you renew the lifters, do a camshaft break-in when you first start the engine.