I have a loud rattling noise when at low rev.

I have a loud rattling noise when at low rev usually from idle til 2600rpm, goes away as the revs build. It sounds like is coming from down around cambelt area, mechanic suspects a dry bearing of some sort but thinks its fine. The noise is getting bit louder and am getting a slight engine miss between that low rev range. I have heard the exhaust heat shield or a separater inside the exhaust itself can make the same noise while reving like a death rattle. Please help me I dont know anything about cars and this is the best way to describe it. Thank you