If it is piston slap it is typically caused by too much room between the piston sides and cylinder wall. If they somehow mismatched the pistons (easy to do in a shop) then it needs to be fixed soon before serious damage occurs.
Take it back to them as they seem willing to make it right. Make sure they drive it with you to hear it.
Take it back to them as they seem willing to make it right. Make sure they drive it with you to hear it.