you're probably hearing the purge solenoid clicking on the charcoal canister. it would explain the 'coming and going' anyway.

Thanks, it does sound like it is coming from that vacinity.

next time you hear it put your hand on the solenoid, you should be able to feel it.

If it happens when the car is not moving you can use a mechanics stethiscope to listen around for it. Get a long screwdriver or prybar and place the poinst of the tool where you think the sound is coming from. Then just put your ear on the plastic part and listen. Keep moving the thing around till you figure out where the sound is coming from.

Be very, very, very careful while doing this though. You are going to be putting pressure onto a unstable object with your head so make sure you are holding it firmly with both hands. If you have lone hair like me make sure you tie it up and out of the way.