Prox, if it's only in the right speaker, the first step would be to switch the speakers to see if a problem has developed in one. While "rattle" isn't the way that I'd describe the sound, it's possible that the amp is clipping on some extremely loud sound in the right channel. I'd think though that your amp should be fine except in a very large room at very high volume.

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Enjoy the music, not the equipment.