The knock is probably rackends worn or it could even be the rack mounting loose - really you need to get the car on ramps and have a look under to locate the fault exactly. Rackends can be had for about $60 a pair - you can install yourself. Could also be the tierod ends or the pinion may need to be re-positioned to the rack (do you have steering freeplay?).
You can get good ones from wreckers for about $100 with a guarantee but you still need to replace plus you will need to read up on what exactly to do and it will take at least a full morning and is va dirty job if the area is oil saturated. You can pay a mech to change it over but figure on about $2-300 labour (that's a few slabs!). Or you can just drive till it gets worse but not to dangerous worse.
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