The best explanation from what you describe would be injector rattle. This is a totally normal idiosyncrasy common to the 1800. This would be as you describe very rhythmic, subtle and may be worse under certain circumstances. A stethoscope with the pickup placed in the one inch gap between the upper fairing and the upper part of the valve cover where the injectors are located will confirm if this is what you are hearing. (In lieu of a stethoscope, a length of heater hose held to the ear and the other end to the suspected source will isolate the noise almost as well.)
If that's not it, you may not want to know what else...
Clutch basket rattle or ACG, although both of these would be more toward the back of the engine... (use the hose )
Let us know what you find...:thumbup:-