2002 Optima 2.7 clattering noise

I have a 2002 Optima 2.7 V6 that just started making clattering sound like a very noisy desiel engine. The car will start up and run, but it has just enough power to move the car. All cylinders still have good compression. The spark plugs are a little dark and sooty. It sounds like the noise may be coming from the rear head.

Any ideas what the problem is or what else I should check?

Thanks.

Found the timing belt had jumped. Fixed the belt and the car ran good, but now as I run it longer, it has developed a loud tick in the rear head. I will either replace the head, or just drop in another engine.

It jumped time for a reason. Might look at the tensioner or the adjustment on the tensioner. Usually if there is a ticking noise from the head, the hydraulic lash adjuster is at fault. A lot of times I can get them to quiet down with a fresh oil change and a thorough thrashing.