rattling on acceleration

It is very hard to express an opinion on a noise with out being able to drive the van and hear the noise first hand. A rattling on acceleration hopefully just a loose or damaged exhaust heat shield but could also be a sign of a bad catalytic converter. It could be many other things but it is best to get it checked out as if left neglected it may result in more serious problems.

Could also be the electronic knock sensor or the block going bad.

On my wife's liberty, I found it was the fan belt tensioner. It only rattled at a certain RPM, right above idle.