Any sound which is an unwanted signal is noise. This noise can be kept to extremely low levels, but when sound is transformed into an electrical signal, noise can be added to it in ways over which we have very little control.
If we play a blank tape we find that the noise level is very low. The noise originates mainly in the recording process. It is, in fact, the erase and bias signals which arrange the magnetism of the tape particles at random so as to produce the effect of noise on replay. Part of this noise can be reduced by good design of the oscillator which produces the erase bias signal, and by good processing of materials used for the production of the tape.