You can save yourself diagnostic costs and possibly being ripped off by going out and buying a stethascope from your local autoparts store. Its much like what a doctor uses but it has a hard tube on the end so you can listen to exact places. Use it to listen around your engine and see if you can locate the noise. Remember harmonics in engines can be easily transfered from one side to the other so make sure you listen everywhere, even if you think you found it check other places and see if its louder. Also listen to each injector, they do tend to click when they fire. If you find noise when pressed to your valve cover slide under your car and press it to each of the oil pan bolts. If its louder under it you might have bearing problems.
Stuff got done.