Cracked flexplate will/can cause knocking noise. Can be very loud sometimes.
You won't be able to see the crack with trans installed.
Flexplate must be removed from crankshaft also.
They usually crack around/near the center pilot hole, or from a bolt hole to a "window" if it's not a solid flex plate.
Crack can be very hard to see sometimes. A very tiny hairline crack can be almost impossible to spot.
Hold flexplate up and tap with a hammer, it should "ring" nicely. If it makes a "clank" sound it's cracked.
Another way to spot a crack is to pour some solvent on the flex plate, just enough to "wet" it.
Then drop it horizontal on a flat surface (concrete floor) from a few inches up. Watch closely and if cracked the solvent will "spray" from the crack. May have to repeat a few times to see it.