I recently bought a second hand bike, that got through quite some repairs and testing at a nearby bike store. They fixed some issues and put the bike into cycling condition, as well as changing the tires.

After some travelling, I have noticed that on very bumpy roads the rear suspension starts making what i would label as "B-series movie bed suspension noise" (niieek, nieek, nieek), the spring seems to be "jumping" too much and provoking the scratchy sounds.

I have also noticed that the rear suspension has some kind of screw mechanism to strenghten or loosen the spring (see pic), and that it's halfway, but I have no idea how to touch it and I don't want to trouble the bike store again for such a simple thing. 

I understand that what I should be looking for is to increase the strength / stiffness of the suspension, to reduce the scratching, at the (relaxing for my ears, worse for my own rear) cost of less shock absorption.

[![enter image description here](https://i.stack.imgur.com/COaz9.jpg)](https://i.stack.imgur.com/COaz9.jpg)