Cyclical tap/light drum noise.

We have a cyclical/occasional noise, like a slow & distant drum beat. It is so faint that the first time I thought "which very-faraway house has the budding rock-stars?" I really had to be patient and listen closely to determine it came from under the hood, and seems more from underneath the car. It is not very noticeable, but it's there even when idling in the driveway and it is repeated every so often. (It's most noticeable idling, I'm thinking since there is no added noise to cover it up.) I'm hoping it is not a cylinder, but I'm sure there is a badly-overdue tune-up. What do you think? What should I expect and what would be reasonable to hear from a reasonable mechanic?