Here's a problem that caused me much gnashing of teeth and tearing of hair before I nailed it. Some of you may find it old news, but here it is: Mastr II station with NHRC controller. Mobile PA, with fans added so the PA would live through nets and long transmissions. Problems with a "humming" noise on repeated audio, low deviation, but annoying. Station mike clean, but hum (sorta like PL tone) on any repeated audio. Lived with the problem for a while, but an overhaul on the bench on other problems (whiskers in the rx front end, etc.) gave me some time to play with it. Turns out it was the fans! I had used 4" 12V brushless muffin fans. The fans were running from the 12V station power supply. When I disconnected them in desperation, the noise went away! I built a little filter from a toroid core and a few caps in a pi-net, but the noise was still noticeable. I installed two 120VAC fans and the problem was solved.
My theory is that the 3-phase oscillator in the fans produced square waves to run the little fan motor. Square waves have lots of harmonics and was radiating them into the rx audio line into the controller. Word to the wise-- be careful of 12V brushless fans around susceptible equipment. John W5EME
