Q: Car won't start but when attempting to start it makes a repetitive "pop" sound.

Hi there. What could be happening is the engine has either a large vacuum leak or the timing chain has skipped a tooth making a camshaft out of adjustment. First look all over the engine for any possible vacuum leaks, included looking at the brake booster for it uses the most vacuum. If there is no vacuum leaks, then try to hand crank the engine over. If the engine will crank free and has no binding, then most likely the timing chains are fine. The other thing to check is the fuel pump for fuel pressure. When the fuel pressure drops to almost nothing, the engine will not start and cause a constant popping sound. Check for fuel pressure on the fuel rail. If there is no pressure to the rail, check the fuel pump relay. If the relay is working, then the fuel pump needs replaced. If you need further assistance with all of a sudden the engine is not starting and begins popping, then seek out a professional, such as one from Your Mechanic, to help you.