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@thor I'd be willing to bet its the powersource. Your speakers are likely using a switching power supply running at 4 khz.. the reason is a few 1) thats typical for guitar amps 2) the harmonics are typical of a square wave, which means your seeing RFI from a PWM like in a switching power supply.
So my top guess right now with like 80% certainty is that it is the power supply to the amp (if its a wall wart its that, if its a straight plug its internal).
@freemo you could be right that it's switching noise. the power supply is internal. these plug straight into the wall. i'm wondering if this can develop over time, because it would be pretty bad to send them out of the factory like this. and i'm noticing that one speaker is making more of the noise than the other.
@freemo based on this teardown, its baby bother, the HS7, seems to use a linear power supply:
https://www.head-fi.org/threads/yamaha-hs7-internals.745768/
@thor So i just did some quick checking. the reason amps tend to show noise at 4khz is because they are tuned to have a peak frequency response right around there (which makes sense as that is where the audio frequencies are).
Due to the harmonics id still lean towards a switching powr supply.. the only other thing that would produce those sorts of harmonics would be an arc.
So if its not a switching supply in your home, and i presume you dont have high voltage in your home, then the next most likely culprit would be your power line transformer outside... it probably leaked its mineral oil and is internally arcing (very common and would produce this particular RFI)
