Knocking sound

I have a 98 Windsor 4.6L that was just rebuilt by a reputable company. When it was reinstalled in my truck there was a knock and somebody said it was loose tensioners allowing the timing chains to make a noise. They replaced the whole timing thing, chains, tensioners, etc.
After I got it backthey said put some miles on it, so I put 3000 miles on it. After the truck warms up I still hear a knock. I have had three other mechanics listen to it and I get a unanimous "piston slap", but these same mechanics say that the rebuilder will "make it right".
Does anyone have any ideas about where the knock would be coming from? It's not a sound like a rod, it's higher pitched than that.