Dodge: 3.3 V6 engine rattle noise, catalytic converters, grand caravan

Question
My daughter recently bought a '97 Grand Caravan with 120k miles. We are perplexed by a rattle noise that only occurs at about 1500 engine rpm.  It appears to come from the upper part of the engine at the front end, ie. not the transmission end. It hasn't gotten worst but is always there whether the engine revs up or down through 1500 but only over a very narrow range at that speed.  Water pump was replaced because it leaked and that had no effect.   Any ideas?

Answer
This noise is common in the v-6 engine.I read a post where a guy took his to the dealer and was told the same thing.They don't even know what it is.My dad has a Dakota v-6 does this same thing.I think it's something to do with the catalytic converters on them.