Anybody getting feedback when using USB for ipod?

I'm getting feedback on certain songs when playing my ipod through the USB port.  It is consistent with specific songs, but not necessarily every time.  I have a customer that bought a 4Runner from me and is experiencing the same thing.  Toyota doesn't have a bulletin out on it or even mention it.

It sounded like rocks getting kicked up by the tires and hitting the underside of the truck when I first heard it, but it is definitely the stereo.  I have the JBL factory system.  I can duplicate it when I need to, but just wondering if anybody else has had it, or more importantly, is there a fix for it?

Thanks,

Speedster