Sound Fluctuation

I have the Entune Premium system and when listening to FM radio the sound will fluctuate. Not in volume but in tone. Something like when your ears are glogged and then they clear out and the sound changes. That's the best explanation I can think of. It doesn't happen when listening to XM or from the USB. I'm not sure if this is a problem with the head unit or with something else. Has anybody else encountered this? I've already done the Kicker upgrade so I know it's not the speakers.

 

My 15 does the same thing, does it when it toggles from regular FM to HD radio.

 

I was thinking that was maybe what it was doing also. I never paid attention, does the radio Indicate when it's in HD or regular mode?

 

Mine does, Little HD radio symbol on the screen.