Can you tell me what kind of radios are being effected - are they all the same type of radio? If so, check it's mod status - especially if they are VHF 20's
If there is local interferance then what others have said is right, it's a job for the ACA, however if it's being caused by other aircraft equipment then you just need a radio tech.
I don't know what aircraft you have, putting 2 & 2 together I am guessing it's S76's. It's quite possible that ageing avionics in each might be getting dodgy capacitors and therefore creating spurious emissions. This being the case, it would have only been a matter of time until you became aware of the problem. Another problem could well be antenna bonding - is the problem worse in the wet?
On the aircraft with the EFIS displays making the noise, have you tried tracking it to a single display, or all at once?