Are you familiar with bagpipes?
The three pipes coming out of the bag that the piper is not actively playing are called "drones."
At pressure, they emit a tunable frequency and when they're out of tune, it can be epically awful.

Some people hear an irritating noise / "musical" note in several aftermarket exhausts on the F6B and the Wing when the bike is at speed at constant RPM, say cruising at 60mph.
Not all people hear it even with the same exhaust systems.

I'm running stock and I don't have it.
I've a sound freq meter app on my phone (I'm a sound engineer) and I have nothing that stands out.
If I ever run into someone with a "drone", I want to ride bitch and take readings when they hear it to see what which freq it is.

Cheers,
Steve