The correct answer is D) dirt on magnet faces. A loud buzzing noise coming from the contacts in a magnetic controller is typically caused by excessive dirt or debris buildup on the magnet faces. This condition can disrupt the magnetic field and cause the contacts to vibrate or "bounce," leading to the audible buzzing noise. The other options are incorrect because: A) Excessive current would typically cause overheating or arcing, not a buzzing noise. B) Bouncing of contacts is a symptom, not the underlying cause. C) Excessive magnet gap would reduce the magnetic field strength, but not necessarily cause a buzzing noise.