1st you should check your spark plugs and exhaust manifold bolts/nuts. If you didn't get one tightened down good either one could rattle.
Next, you could either get an automotive stethoscope or a long screwdriver, prybar, etc. With the engine running start touching different areas with it, watching out for moving parts (fanbelt and accesories) and plug/coil wires.
I recently replaced the cap and rotor on my wifes cherokee and didn't get the cap down quite all the way and noticed a ticking and tracked it down using a small prybar.
Hope this helps,
Dustin