The PWM Drive (pulse-width modulated drive) to motor power conductors are typically the most intense noise source in a system. Proper implementation of shielding, grounding, splicing, and treatment of excess cable is essential to reducing noise in your system.
Wiring diagrams happen to be a perfect vehicle for carrying the principles of technicians beyond nuts & bolts. First, the simple act of color-coding helps to bring out the true wealth of your knowledge and is an excellent step in diagram analysis. Beyond that, it is an amazing tool for developing the awareness needed to get on the road to becoming an expert learner.
The unshielded conductors radiate an electric noise field that couples capacitively with adjacent wiring. Stray capacitance at A & C cause ground currents to flow creating a magnetic noise field that couples inductively with adjacent wiring.
Think of the atoms of magnetic material as an unruly herd of cattle. Running electric current through the material will polarize these atoms, creating the magnetic field. But as I mentioned, this is an unruly herd, so it takes time for the current to bring all those atoms into formation.