Most common. Usually a ground loop causes hum. A ground difference of potential between two audio sources. Bad ground connection. If both on the same good ground that can be one solution. Interference from magnetic field at 60Hz. Bad power supply filter capacitors, and many more. With a turntable I have seen rumble from loud audio feedback though the vibrations going through the vinyl record into the needle then amplified as looping hum.
What causes the music to hum and distort when playing a vinyl record
I have a pro ject new turntable in a stand in the corner of my room connected to my Yamaha 2080 also connected to my anthem mc20. Cables are new tributaries and ground wire connected. Sounds good but when I turn it up halfway a humm starts loud and starts to break up the sound and I have to run to turn it down. Is there any direction I should look at first as to what is causing this. Thanks