What octane of premium do you use? Anything greater than or equal to 91 can be called premium. I avoid running anything below 92 in my Talon as it is very sensitive to knock (93+ normally). First things first, try increasing the octane level of your fuel. This predetonation can cause catastrophic failure, if it starts pinging let off of the accelerator before you damage the motor.
What I wonder (as I do not know anything about SL600s) is why isn't the knock sensor picking this sound up, telling the computer, and retarding the spark? Even if the fuel mixture is running lean or whatever could be causing the knock, the car should see the problem and adjust the timing so nothing is damaged.