My Hunter ceiling fan after years of silent work has started making a hum. Any ideas of what is causing this?

Thank you

Electric devices hum... are you ready for the answer?

Because they don't know the words!

Seriously, you can try blowing out the motor with compressed air. It may be clogged with dust and overheating. Or, and this is a stretch, you can check that all of the splices are intact and not causing any resistance. Or the fan may just be nearing the end of its life... these things can run forever even though they might get progressively louder.

What model Hunter fan is it? How is it controlled?

General ceiling fan troubleshooting and help FAQ- Ceiling Fans N More

PS, first thing to check is loose screws.