Normally I would expect such a problem to be a radio receiver problem not interference but if you have tried two radios with the same result then seems likely not. A remote possibility given you have mentioned a Sangean radio is that you have it in SSB mode (or the BFO turned on on older models) if it has that function, but even so it should not give a tone over the whole band, just around the stations.

If the problem is coming from your property, even though you have checked, then wandering down the road with the receiver will make the interference quickly disappear - if it is interference from a remote source then you will get it still even kilometres from your property. If that is not the case I would check with neighbours who will also suffer the same interference  if it is not local to your property.

If it is experienced on their properties too and when you wander down the road then I suggest that your next move is to speak to the Radio Spectrum Management people at the Ministry of Economic Development.  http://www.rsm.govt.nz/cms/consumers/reception-problems/how-can-i-register-an-interference-complaint explains how to register a complaint. They will probably quiz you quite alot over the phone about it as most interference sources are local to the person's property or is a fault with their own radio/TV/etc in which case it is your problem to remedy, so be ready to explain in ordered fashion all the checks you have done.