Knocking noise

I took my car to the local garage for the front brakes to be repaired, when I got it back it kept making a knocking noise from the passenger every time we went over the smallest of bumps in the road like something was hitting the ground. So I went back to the garage and he spent a hour on it and said it was all okay, and the noise did not happen anymore. Then I had some repairs done yesterday and again and it is making the noise again. What can this be? It seems odd that whenever they repair it the noise disappears? Also, I thought it was a bit steep paying £50 for radiator leak stop fluid? Is this too much money for this?