[jimjaz]

Possibly engine timing like Jim A describes. You will need a timing light to check this. The distributor most likely has a vacuum line coming off the back side of the manifold and goes to the distributor- this provides for more vacuum advance (less throttle/gas) or less vacuum (more throttle/gas). The distributor also has springs under the cap and are "calibrated" to have a specific mechanical advance at a given RPM to advance or retard mechanical timing, or advance. If you don't have a timing light, then;
1- take off the valve cover on one side and check for anything in the valve train that may be loose, like a rocker stud or the nut that holds the rocker in place which would cause the rocker to make a lot more noise than the others. I would say that given your admitted lack of experience, you would be wise to elicit the support and knowledge of either someone you know, or a shop in your area that is capable of diagnosing a big 'ole V6 that is definitely old school.
Good luck! - hope that it is something that can be identified and resolved without emptying your wallet.