Q: Car won't start I can hear it trying to crank. I've just replaced the alternator and battery it just won't turn over...

This suggests that the motor have run out of oil. When the oil pressure gauge reads zero, this means that there is not enough oil pressure inside the motor to properly lubricate engine components properly. When this happens, you may experience catastrophic engine failure which is most likely where the rattling noise may have come from. When this happens, depending on the parts that have failed, this may also cause the car not to run at all as you have experienced.

Unfortunately, without diagnosing this there is no way to speculate or tell what may have happened. To avoid any unnecessary repairs, I would recommend having an expert from YourMechanic come to your location to take a look at your vehicle to diagnose your no-start problem.