A knock is usually a bush and/or something else in the steering, not necessarily the power steering... but could be the rack as you have said.
Check for any free play in the steering, while motor is off, giggle/turn the wheel a little and see if the tyres move. There should be almost none... anything more than 5-10 degrees means it's not good.
I would check the link ping, front/read radius rod bushes, zbar bushes, tie rods and top strut mounts... After that it's the wheel hubs, bearings etc. etc. etc...
Disclaimer: I am no mechanic or suspension specialist so seek professional advice from a qualified specialist... 