Rattling noise coming from the engine

I'm sure some of you remember that I just spent $1,200 last month for a brand new intake manifold job.Now, 3 weeks after that my 95 Jimmy is making a rattling noise. We took it to the dealer and they LISTENED to it, then called me up and said it would need a brand new engine, the cost would be 4,500. We pretty much told them to go screw themselves because even I know that they were trying to screw me over. It's a 1995 and doesn't even have 95,000 miles on it!

We talked to the service manager and he told me they had given me faulty information. He said it was in great condition, so hee listened to it himself and said there could be crap on the pistons, so he poured some stuff in the engine or something and let it sit overnight then started it up and it was still making the noise. They didn't charge me anything, so I picked it up and took it to a local mechanic that we trust and have been doing business with for years. He listened to it and said it could be need a new crank case(?) or that it could be a cracked piston or could be a control bearing(?). He even said that they could have dropped something in it while they were replacing the intake manifold. After sleeping on it, I gave him the greenlight to take the engine out and look at it to see whats going on inside. I'm waiting to hear from him but does anyone have any idea what it could be?

I was advised not to drive it much but I when I picked it up from the dealer and drove it to the local mechanic (20 minute drive) it ran fine, just made that noise. The noise itself isn't really loud or anything, but it is very noticeable.

If it does need a new engine then wouldn't I be better off just buying a rebuilt engine? Arn't they just as good as a brand new one? Especially since its a 1995. I'm not too smart on cars so any information and insight would be greatly appreciated. Thanks :)

A rattling noise from whereabouts ?