Hi
My son has an ST170 and the steering was very stiff and knocked when turning off the center line especially in cold weather.
Luckily I know a guy who runs a steering rack overhaul company
I did as he said and changed the fluid removed the ressoviour to clean out thoroughly which helped slightly
The fluid should be the green type like they use in Citroen suspension
I the end I had to have it overhauled
Apparently its the seals in the spool valve that go so it sends pressure both ways until it gets moving hence the clonk .
Its not too bad to get it out but have to drop the back of the sub-frame down.
You need to align the sub-frame when it goes back up
There is a special tool but a deep socket of the right size works.
Best of luck Tim.