Hi,
There are a couple of possibilites that can be responsible for the grinding sound:
If the brake pads that were used are semi-metallic, there may be a large chunk of metallic material in the pads that is contacting the rotors when brakes are applied. I have seen many semi-metallic pads that will have fairly large chunks of metal in them instead of the finer pieces that dont create noise.
A sticking brake caliper piston or caliper hardware that prevents the pads from releasing contact with the rotor can cause the pads to wear out in a few thousand miles, which could result in the pads being down to the metal backing plate and thus causing the grinding noise.
Obviously, the wheels need to be pulled and the pads removed to first check if they are prematurely worn out, or if it is just a case of a large piece of metallic material in the pads causing the noise. If this were the case, a new set of pads would take care of the problem.
If the pads are prematurely worn out, then the calipers and caliper hardware need to be inspected and tested to see where the problem is in order to repair
Automotive Repair shop manager
20+ years as a Automotive Repair shop manager and technician
First, make sure you take it when you can stay with the vehicle so that you can insist on them taking you out to the vehicle and showing you the their findings. This is a right you have.
Now, as to how to approach it- tell them of the grinding noise and that you need the brakes re-checked, that you think the pads are making noise due to the metallic chunks in them. Do not mention the possibility of the calipers, as I will explain why below.
If the pads are responsible for the noise, then they will be covered by their warranty and will be replaced for free.
If the calipers or caliper hardware is responsible, this is a separate issue and as such you will be responsible for paying for repair- typically new calipers will run $300-350 parts and labor. This is why you dont bring this up as what you think may be wrong. If you do, then it is too easy for the shop to agree that, yes its the calipers and sell you new ones without every doing a honest inspection. If the calipers are at fault, they will find that out when they do a proper inspection