How to reduce an audio system noise in a way which is similar to a human touching one of the system's components?
It "sounds" like a ground loop or an impedance mismatch, but if everything is properly grounded, then a ground loop seems unlikely, but then again, if everything were perfectly grounded, then you touching anything should have no effect.  In either case, a ground loop isolator should help significantly.


  A ground loop isolator prevents interference in a ground loop circuit. A ground loop circuit is one in which two or more circuits are connected to the same ground wire. Ideally, all circuits in a ground loop circuit have the same voltage potential. However, if the ground wire has significant resistance and current, the voltage of the second circuit will be slightly lower than the first, causing a voltage difference that makes the ground wire no longer have a ground potential. This is known as interference and can be hazardous, inefficient, and a nuisance in many electrical systems.


Although they are not very expensive, be aware that there are different ground loop isolators for audio signals and video signals, so be selective when shopping.