2004 Chevy Impala Knocking

If the knock is only on startup or shutdown, or reving the engine, it is possible they did not properly tighten the torque convertor to flexplate bolts.

Check the coolant level and for any leaks.

In any case if this started after the transmission removal and install, I would take it back to the transmission shop and have them fix it.

Thanks a bunch u practically describe what is going on with my vehicle, it knocks when I start, shutdown, and rev up the engine. When I first heard the knocking and brought the car back to the shop, I though it was the engine and the mananger held my car for a week steading telling me it was the engine and when I asked him exactly what was wrong he still couldn't tell me. So as I was waiting for them to pull my car around one of the mechanics informed me that it is my flywheel or something about some bolts. I really appreciate u taking the time out to answer my question.

All they have to do is to remove the inspection plate and tighten the flexplate to torque convertor bolts.

Unless they stripped the bolts or the threads on the convertor, in which case repair would be on their dime, not yours.