No, Mr. V, they are not adjustable. The valves have hydraulic caps that sit on top of the valve stems, and oil pressure is used to keep the valves at attention. One of the few problems with the 1.8 is that these caps get weaker over time, especially if they get any gradeaux (sorry, Louisiana term for stuff, pronounced, grah-dew) in the hole that allow the oil inside the cap. You might try varying the oil viscosity to see if you find that a thicker, or thinner, oil would quiet the caps. You will probably notice that it is worse on start up and when nearing an oil change (as the oil breaks down). If it happens on start up and goes away as the car warms, don´t worry about it. If it continues, however, you might investigate replacing the offending caps. A full set is not cheap, and replacing them is not for the beginner, but if you are somewhat capable, and not afraid, it is not impossible for the novice.
When I did my rebuild, I decided to go whole-hog and make the switch to solid lifters. The advantages are quicker reaction time, more valve extension, much lighter weight, and no lazy lifter syndrome. Expensive? Yes. Worth it? Probably not. Just wanted to do it? Yes, since I only planned on doing this once, I wanted to do it right.
Best of luck.