Hi, I've made an audio circuit that gets quite warm so I want to add a fan to it. I've tried a 12V DC brushless fan but it causes audiable interference. Is there a way to stop it?

I have tried:-
Wrapping the fan wires around a toroid with no effect.
Placing a diode backwards accross the wires. This stops the fan from working.
Running the fan through a separate regulator. No effect
Adding capacitors. No effect or stops the fan depending on the configuration.

Unfortunately I don't have the option to run the fan from a completely separate supply. Is there anything else I can try or maybe a different type of fan?