1999 Chevy Venture Loud Knocking, loss of power, can't find

Engine Mechanical problem
1999 Chevy Venture 6 cyl Wheel Drive Type unknown Automatic 113000 miles

My van was running fine for the most part and then one day I could hear a faint flapping sound while driving. I assumed I had something caught in the tires. This lasted for about a week and then the flapping sound turned into a popping which grew louder as I drove. The louder the sound got the less power she had until we were topping out at about 30mph and it sounded like large rocks popping around in the engine. It also seemed not to "shift" as we drove. The mechanics listen to it and said I need a new engine. We pulled it apart and the rockers were good, the piston rods are fine, and there are no flakes in the engine oil to suggest an issue with the cam and cam bearings. In short we can't find any reason for the problem. Where else should I be looking. My gut tells me its a cheaper problem than an engine replacement. But I need some where to start. Thanks