2005 Trans problem

I just bought a 2005 T&C Touring with 52K miles. Lately it seems when on the highway going up a hill when the van downshifts it seems to shudder, Seems to happen around 68 mph. I also hear a loud knocking like something is slapping the trans housing (much louder than say a piston knock). The feeling that best describes it is like a 10 speed bike where the chain can quite get to the next sproket and keeps falling back. No check engine light or anything. Any thoughts?