Motor mount

My 99 gtp is seems to be lacking power, it has 80k on it. It is always run on 93 octane and is in great running condition. When i put it into gear it makes a kinda loud vibration sound inside the cab when my foot is on the brake and the car is not moving. Is this a bad motor mount possibly? and if so would it be able to cause false knock retard? I have heard of many things that can cause false kr and was thinking maybe this could be it since my car will barley even squeal the tires. Any input would be appreciated. thanks