Compressor issues

The closer you are to the work surface, the finer the line will be.  I would stay in the 15 to 25 PSI range and experiment. Much will depend on the type of paint you are using, and how much you have thinned it, and how fast it dries. Also, remember, the lower the air pressure, the less the paint will be atomized. The point is, in case you missed it, is you just are going to have to experiment :-)

Sorry, not sure I understand fully your compressor issue. Not enough to give any advice anyway.

Sounds to me that you got a compressor without a tank and what you see (hear) is normal operation. My HF ones does exactly that.

Fine lines are more a function of needle travel than cutting back on pressure though the latter can help depending on thinness of your paint.