One or more of your phono section tubes has gone microphonic. I have a WPL20 that exhibits the exact same problem when one of the phono tubes goes wonky. Tap each with the eraser end of a pencil with the volume set low and you will hear a hollow metallic bonk when you hit the bad one(s). You may notice that your treble is less peaky at high levels once you change out the problem child.
Good luck,
Adam
Good luck,
Adam