......or even on the mains lead for the extractor, as close to it as possible. Often with EMI issues, prevention is better than cure: if you can stop it radiating, then you don't need to shield from it. Does the motor in your extractor make quite a howling noise? If so it'll be a high speed series wound motor and the interference is likely from sparking on the brushes. There are suppressor modules (resistor + cap) around that normally ought to be fitted on a quality motor of that type.