The noise is definitely from the resistor as the CC is outside in a shed. The CC is a Trace / Xantrex C40 which is PWM'd, it's what's causing the electrical noise coming from the resistor, is this adjustable as DamonHD suggests? There's nothing I can find about it in the manual.
Using paired cables to each resistor might help. This can reduce individual strength and reverse flow can cancel the field. Paired conductor insulation will also deaden the vibration.
Sorry, what do you mean by 'Paired', 2 cables in parallel or cables stripped of their insulation? I have the insulation stripped back about 6 inches.
I dont have any hot water system in my cabin so a water heating element is no use to me unfortunately.
I'm going to try building a resistor from thin Nichrome wire and see if that eliminates the noise, hopefully it wont be able to resonate as easily as the thicker stuff...