Well, you could get something which measures sound and take readings where you hear the hum and where you don't.
I'm not sure how far that takes you - beyond reassuring yourself that the sound is really there. But if you went a step further and tried to map out the area (including your flat) where the hum can be detected then you might be able to locate a central point and look about for a possible source.
I'm not sure how far that takes you - beyond reassuring yourself that the sound is really there. But if you went a step further and tried to map out the area (including your flat) where the hum can be detected then you might be able to locate a central point and look about for a possible source.
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