RE: 4.3 engine knock
That doesn't sound much like an engine knock. It sounds like that maybe you have a bolt loose on your torque converter housing, which will cause a knocking sound but not cause it to slip under accel. That doesn't exactly mean its not an engine knock, but most of the time an engine knock will be constant at all speeds.
Generally, new O2 sensors and a new cat will not cause knocking, it may make it more noticable because the engine will run with a lot less stress, making the sound more audible. You'll want to start with servicing your transmission.. When you take the tranny pan off, there will usually be a magnet in the bottom of the pan that collects metal dust and shavings, if that is coated with a lot of debris, clean it with parts cleaner or mineral spirits and finish the service. If its a rediculous amount of metal, it can indicate whether you have a transmission problem. Then change your oil. The magnet on the oil plug will pick up a lot of metal shavings if you have a lot of friction.
If neither is the case, you'll want to rebuld the engine. Then if the noise is still apparent, rebuild the transmission and go from there.