"He said you can hear it doing something."

That's pretty vague, do you hear a ticking that creshendos with the engine when you rev it? If so, it sounds like loose valves. Just check the valves and if they are not in spec. You'll need to adjust them on both cams.

I'm thinking your gas millage has to do with something else, when's the last time you've done a major tune-up?