Knocking noise

Play in the water pump shaft is not normal. Also have you checked the crankshaft pulley for play as they can delaminate and make odd noises. Also I once had a huge nail in a tyre that didn't leak air but on every rotation it would just touch the wheel and make a clanging noise and that took an age to find.

It's as easy and cheap to replace the water pump complete instead of trying to repair it.

Yes. Radiator top hose off at radiator end. Release tension on drive belt by loosening the alternator. Unbolt the radiator shroud from the radiator and the fan from the pump and remove both together it's a bit fiddly but the shroud is pliable enough to avoid damage or if you prefer remove the radiator as wel it will make more space but isn't absolutely necessary. Remove water pump drive pulley and then you can get at the bolts to remove the pump. Some of the bolts are different lengths so remember what holes are which and off you go. You will need fresh coolant to refill the system when you are finished. Easy. Unless you have difficulty differentiating between a hammer and a spanner in which case a good garage will probably only charge an hour or two labour.