What would cause you to lose power and something knocking while going uphill or accelerating Knocking stops after you get up the hill or letting off gas?

What would cause you to lose power and something knocking while going uphill or accelerating Knocking stops after you get up the hill or letting off gas?
You did not say what engine you had, so if it is a stock motor depending on the year and the compression it could be bad or low octane gas. if you are using good gas, try backing the timing down till doesn't knock under accelloration or under a load as in going up hill.