If you want to get into the finer points and technicalities, the noise generated from the gears inside a full metal geared servo could also be considerd a noise source that may cause interference on a really bad day. They do radiate to some extent as well, but your system will have to be mighty uppity to have problems with them. Unless you have some really bad servos[] I have to admit there are some systems that require shielding a metal geared servo for just that reason, noise from the gears...