Staticky music when playing from iPhone?

Noticed this today... I plugged in my iPhone 4 via USB (using genuine Apple USB dock cable) and played some songs through it. I noticed that occasionally (every 5-10 seconds), I would hear a static "pop" sound. It's faint, but noticeable. I thought it might be the volume too high so I turned it down, turned off EQ and DSP modes, and tried different songs. Same thing, and the static would occur in different parts of the song, even if I rewound and played it again.

But if I play the same songs via Bluetooth streaming, sounds just fine.

Is this happening with anyone else?