I once had a rattle, but on a speaker, the left main, and it rattled the very most with one of my display calibration discs (that has a collection of audio sweeps). I had screwdriver out, ready to start pulling off drivers, until I noticed it was speaker wire that was touching the speaker! Pulled it away, rattle gone.
Your connections are secure, and these cords are not in contact with the spikes, cabinet, etc?