My car makes a sound in the engine, like a bump bump bump...

Sounds like an engine knock from gasoline. Using the higher octane of the additive and premium gas makes it go away. Perhaps a knock sensor has gone bad, and is trying to overcompensate for lower-octane gas. You may also need a tune-up.

Thank you, for your help. I brought my car in for a tune up and the macanic said one is not needed. A knock sensor, I have never heard of but will check it out...Thanks again