Creative Zen X-Fi strange bandpass

[pdp11]

Today I had analized a strange outupt with my Zen X-Fi portable player.
I tryed to use as a sweep source for some test, because I have some wave files containing sinus samples and a sweep. The Zen can play without any hum problem caused by the pc ground loop. the amplifier is a prototipe and for now is connected to earth ground.
I some problem, a very strange noise in the new input stage that I'm testing.
I attached my muvo X-Fi to the oscilloscope and I found this.
This is the headphone output signal, attached to 1k Ohm. The oscilloscope probe is attacched to the resistor.
The freq. had changed slowly from 6 MHz to 5.5 MHz.

It's a quite strange noise on the output.
I have seen many dac noise, many interfernce, but it's the first time that i see a big and clean 6MHz sine in an audio output.

PS: I'm sorry for my brutal english