I have been getting a short period of knock under light accelleration when in 5th or 6th. example cruising down hwy at 65 go to pass rather than downshift just push down on gas pedal and I will get about 3 seconds of knock that then fades out. I have never put anything but 93 octane in the tank. It is worse with red key than silver but both do it. any ideas. I have tried a month on Exxon and then a month on Shell Vpower with no difference.