Q: My car is making a knocking noise and there's a fuel leak

You need to have the fuel leak fixed ASAP. A fuel leak can cause a fire and at the very least it will reduce your fuel mileage. Fuel leaks are no different that a leaky water pipe in how they leak. Difference fluid and more pressure, but leaks are leaks. You first need to locate the leak before you can know what needs to be replaced. I recommend having a fuel is leaking from the gas tank inspection. Include a note to the mechanic, if you choose one from YourMechanic, when you book this appointment letting him know you are not sure where the leak is coming from.

The knocking noise is another issue. There are dozens of things on a car that can cause a knocking noise. Although, with the given description, I could say that maybe there is a drive line problem. This would be an axle, drive shaft, or transmission issue. Locating where the knock is coming from should reveal what the issue is. If the noise begins when you put the car in gear and attempt to go forward, one of these is most likely the cause. I recommend having a car is making a noise inspection.