Relays on the main board will hum pretty loudTransformers can get real loud and the microwave will still heat.
Before I go any further I need you to understand that the capacitor in a microwave stores up to 6,000 volts. This voltage drains slow. The first thing when serving one is to discharge the capacitor. If you don't know how to discharge a capacitor, then you don't need to be repairing a microwave.
The capacitor can and will KILL YOU. Many people and technicians have been KILLED by the capacitor. Even unplugged for days or weeks it can hold 1000's volts. Have a qualified technician diagnosis and repair it.
I've used this method for years to diagnose microwaves that don't heat. We need to know what the amperage is. We set on high to cook. The power cord plugs into a line splitter and splitter plugs into power outfit. Clamp the amp probe on to the splitter. Place a cup of water in microwaves, set to cook 1 1/2 min. How many amps did it pull? If it pulled:
Less than 2.0 amps
problem is in the low voltage circuit. -PCB, door switches, fuse, thermostats etc
Between 3-9 amps
Problem in high voltage circuit (capacitor circuit) test the magnitron
10-14amps. All operating properly.
Good luck-Be safe.