Rack And Pinion Bushing
A common problem on the MKIs is that the RH bushing inside of the rack and pinion goes bad, leading to a thunk noise. It can be cheaply replaced for about $6. Do not let Toyota charge you for replacing the whole rack and pinion. Remove the boot and unscrew the inner tie rod from the rack. Turn the rack all the way to the left, and then snap out the old bushing and pop in the new one. There is a lock washer you will want to replace also.