Sue, I'm not sure about your engine size, but the 3.4 engines are commonly known for intake gasket leaks and the 3.8 is known for the intake manifold going bad. If the noise you hear is a fast tap or clatter near the top of the engine, it could be valve noise which would require you to have some cylinder head work done. Some noise can come from a worn camshaft. It's also a tapping noise. If the noise is a steady metallic rap, it may indicate a rod/rod bearing. A quick steady tap may indicate lifters or cam lobe, while a clatter may be loose/worn rocker arms. A low rumble or knocking noise could be caused by a bad rod bearing on the crankshaft. Here are a couple of sites that will tell you a bit more:

• http://www.remanufactured.com/Engine_Noi...
• http://www.aa1car.com/library/engine_noi....

As you can tell, there is a multitude of problems that can come with that noise. Try to troubleshoot a little at a time. Check your plugs and see what they look like at the business end. You can get a lot of info from that also make sure your oil and coolant levels are okay. Maybe get somebody to do a compression test and a leak-down test to see if your valves/rings etc. are okay. What do your exhaust fumes look like? White smoke? Black smoke? White smoke could indicate that some of your coolant gets into the combustion area, while black smoke suggests excessively burning oil. Good luck to you, and I hope this helped a little.