Some metal film resistors use steel leads. Metal Oxide Ruthenium Oxide is the conduction path as particles contact each other. However, being so irregular like a super ragged edge creates much noise. These generally use steel leads. Carbon Film Carbon particles is the conduction path as particles contact each other in a binder.
Mechanical movement is possible with resulting instantaeous changes in resistance.
Current changes paths as minute mechanical changes occur. Crackling noises occur and resistances can change at any time. Moisture is also a problem. Noise from these resistors is the highest, and as seen in the above tests, the worst compared to the highest quality and expensive resistors for sonic quality. Some claim to make excellent carbon resistors, but the fact remains that carbon particles touch each other.