2001 Dodge Stratus 2.4L engine knock

I have a 2001 Dodge Stratus 2.4L that has a slight knocking sound when it is at an idle but when you accelerate the knocking gets worse. This is a replacement engine that is only suposed to have 30,000 miles on it. The motor has a waranty and I have only had the car back for a day, my mechanic said that it was nothing to worry about but that doesnt seem right, what could be wrong with it?The original engine broke a timing belt and my mechanic said that the engine was no good? I need some help because I think that the mechanic is ripping me off.