What would cause my gas gauge to tell me I have a quarter or a tank and go to e?

What is your question?

This problem occurs before I changed my oil then after changing after a few drives it read accurately. Took the truck on highway got a knock sensor error and the had gauge is back to acting up. When I start the truck it reads the current fuel level but after a minute into driving it slowly goes back down to e. I also have noticed a rattle in my gas pedal when accelerating it occurs around 30 mph