Are you actually hearing the pre-detonations (pinging, sounds like popcorn popping) or is this because you know the knock sensors are pulling timing? In other words, is it possible you are getting false knock from somewhere in the drive train? If you actually hear the pinging, it is obviously not false knock.
Check the FPR like Justin suggested. There is a small diaphram inside the FPR that can rupture and reduce fuel pressure. If it ruptures, fuel will be sucked in to the vacuum line that feeds the FPR. If you pull the vacuum line off you will smell gas in it if the FPR diaphram has failed.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)