Hi alll

I just paid over $5K to have my fuel pump replaced. They said it was in pieces and overhauled it. Then they have just rung and said that as they went to drive it out ready for me to pick up they have heard a new knocking noise. they think its coming from the 3rd cylinder. They said it just started and it isnt related to what theyve just done - but i just find it so weird that the noise has never been there and it suddenly appears on the day im picking it up after them having it for a month and never saying a word before now. Is there anything people suggest i ask them to do?