how do i cure a diesel knock on a ford focus?

I've recently had re-conditioned injectors fitted on my ford focus 1.8 tdci as the old ones were on there way out.Now i have what is referred to as a diesel knock which occurs on accelleration,more noticeable at low speed & get quieter when at high speed.this knock only occurs when accelerator is pressed can you advice on how this can be resolved as it wasnt present before car went to garage to have injectors replaced