It may be "pinging". This can be caused by the knock sensor not working correctly, or insufficent EGR. It could also be caused by the engine running a little too hot, or even something like a spark plug not correctly torqued, or even cylinders that are getting carbon deposits. At the same time without me actually getting to hear the sound, that is only one possibility based on when you say you are getting the noise. That light throttle application at cruising speed, that creates a light acceleration is called "crowd". It's common for pinging to occur under those conditions.

OK--you lost me at EGR--what the heck is that? Bottom line, should I be concerned especially since this weekend I could not reproduce the "clicking" noise? Also, maybe it's just my imagination, but it seems to be sluggish on acceleration as well. Any connection between the two? Please talk in girl talk--you know--as in this is the engine, this is where you pump the gas--. Really though I appreciate your response, just don't know if I'm looking at an expensive problem here!

Take it to a trusted garage and have them run a set of diagnostics on it. There may be more wrong that a malfunctioning EGR (exhaust gas recirculator).

Additional Information: An exhaust gas recirculator is a device that admits a controlled amount of exhaust gas back into the intake manifold. Sounds crazy, huh. This thingy was added to reduce the formation of nitrous oxide becasue the engineers couldn't think of any other way to do this at the time while remaining under the then current design retraints. It was recognized that it was going to be a problematic device since it is a valve that must handle hot exhaust gas right out of the cylinders. Issues are high heat and fouling by exhaust particles.

Nitrous oxide is formed in the combustion process by temperature and pressure: The higher the pressure and temperature, the more nitrous oxide is formed. Admitting aome exhasut gas into the mixture casues it to burn at a lower temperature and lesses the pressure as well, but you know, this also lessens engine efficiency and power, so is it a good thing - maybe. That's the rub: as the fuel mix is adjusted toward best efficiency, nitrous oxide also rises. The EGR is supposed to meter exhaust gas into the intake under specific conditions. There is none at idle and varing amounts at other times.

By adding the EGR, it was found that the spark timing could be advanced toward the same settings that were in use befoe the advent of polution controls. On some engines, a knock will occur if there is no EGR function under this set of conditions: Engine fully warmed up; part throttle while at a moderate speed, say 35mph; then encounter a small rise in the road surface that requires slightly more throttle opening to maintain speed.

There is another source that makes a sharp, almost clicking sound, and that is a blown exahust gasket. When the leak is small, it may be quiet at idle and low power output, but will let itself be heard as soon as more power is called for. This sound is distinctive and sounds quite different to a knock or valve click.

What's fuel knock anyway? An internal combuation engine runs the best when the fuel burns evenly and casues the pressure to rise in a smooth manner. Knock occurs when nearly all of the fuel explodes at once producing a sharp, explosive pressure shock instead of a smooth 'push'. Many hours and much money has gone into knock research, and still does today. Its a difficult thing to quantify since the process takes place inside the closed chamber above the piston. How does one get a look inside the combustion chamber? And how does one get a good picture through the flame? One of these tools is a special sparkplug that is fitted with a pressure sensor that is built to withstand the rigors of the combustion chamber.

Just to follow-up: My check engine light came on shortly after I posted this message. I took it into the dealer and they diagnosed the error code as O2 sensor. I had it replaced and the light is now off and I no longer hear the clicking noise. However, I just recently returned from a short trip (250 miles) and it seems that the engine does not have the power accelerating as it used to. No noise or anything, just seems to be sluggish. Any more suggestions?
Thanks for the previous responses!