Someone wants to trade me their '99 Pontiac Grand Prix. Problem with that is he says there is a rattling in the motor. He thinks its a bearing. Any ideas on what it may be or if the trade is even worth it?

Well what is the trade? It is very hard to tell you what that ( rattling ) is with out actually hearing it. There could be a dozen things wrong with it. You really need to have a good tech check it out and let you know what kind of problems your looking at. For all you know it could just be loud lifters, This is pretty common on the 3800 with alot of miles.

If its a bearing it needs an engine. With the amount of labor required to do an engine swap its worth very little if anything. Even if you can get an engine cheap (which you can) and are doing the work yourself, you're going to put a ton of time and money into this car and it will still be a 99 GP. Price it accordingly.

The 3100 and 3400 both have piston slap when cold. The piston skirts are too short and they wobble until warmed up.