550coupe

Under heavy load, my cls pings/ rattles (detonates) for a brief moment. Sounds like marbles in the exhaust. I've noticed that it did this on occasion even when it was still under warranty. When I took it in they said they didn't hear anything and that Benzs ping a little.
Anyway I bought the story and now 3 years later, it's even more pronounced. It's like I'm running low octane but I always run 91. Could it be the knock sensor going bad? I've used fuel system cleaner with no luck. Some say it's carbon build up and I just need to drive it hard...? Any input would be appreciated, Thx.