Van has loud knocking sound when idiling sort of goes away when reved up?

my camaro did the same thing...at first i thought the engine was going...it turned out to be the torque converter on the transmission. which would be good news in your case....way cheaper than buying another engine. take it to a tranny shop they will tell you for sure.

This could also possibly be an oil pump problem. When the van is idling, the oil pressure is very low, but when you increase the speed of the engine, the oil pressure increases,,,,,in your Words, "sort of". Needs to be looked at by a good mechanic. If that oil pump quits, you will seize up the engine.