Engine knocking

Does it go up with engine speed ? Sound like a hammer inside engine? If yes then a rod bearing is faulty.

my 2001 chevy malibu has been knocking for around a yr and i took it to get looked at the other day and they said it was a piston hitting the head and that there was no way of fixing it my ? is what would cause this and is there any way of repairing it

First, how did they determine a piston was hitting the head? It could be a rod or main bearing, cam bearing... Also, it could be caused by carbon build up on the piston that there are cleaners for.

One day I was driving and I made a sudden U turn and it sounded as if something had snapped in the engine. The car then became very shaky and hard to accelerate. The engine light was flashing and it was making a loud engine knock. We put the car the car in reverse and and it made a horrible grinding noise. The car still starts and there are no metal pieces in the in the oil, and one lifter is not stuck. Can a stuck lifter cause this, or has something else broke and cause engine damage?

It as almost that you describe a broken axle shaft, the reason I say this is when you put the car in reverse it make a horrible grinding noise, A broken axle shaft can do this, As for the check engine light to flash an axle shaft broken wont do that, So look at your axle see if they are intact , This can be your problem

You will also have to get a scan done to see what has caused the check light to come on.