My Envision Premium II has a random noise that happens sometimes when the vehicle is traveling up an incline in the road. This seems to happen when when giving the engine slight gas but not enough to cause a shift in torque (? shift to higher RPM). The noise sounds like a shimmer or shake. I thought this might be pinging, knocking caused due to my use of 87 octane gas but I am now using 91+ octane gas and the noise continues to show up on occasions. I would like to know if anyone else has experienced this problem.