I recently had my stock head unit replaced with a Kenwood KDC-MP445U. One of the first things I noticed is that has an annoying hum, roughly 130 Hz to my ear. The hum does not scale with the volume, it's most audible when the volume is at zero, or if there's a relatively quiet segment in the music. It's not apparent if the music is high, but it's really annoying between songs and if the music hits a low. It doesn't appear to change with engine usage.
The installer claims it's not a ground loop because it's constant in volume. His best theory is that it's because the KMP445U's output is high compared to what the stock amplifier is expecting. His solution is to re-wire the car speakers so they're directly connected to the head unit, which is somewhat expensive in labor. My problem is I'm having trouble is I think the hum is consistent with what I know about ground loops, which would only require a ground loop isolator to fix. Of course, I could be mistaken.
Can anyone shed some light on this?