There are a few reasons for brake pads grinding after replacing them with new pads:
- The pads are new – When your brake pads are new they need a little bit of wearing in so they can function normally. This is commonly referred to as bedding in. It’s kind of like getting a new pair of sneakers, at first they are a little bit uncomfortable, but with constant wearing in the next few days they loosen up.
- Rust or debris – If there was some dirt or rust on the rotors while you changed the pads, the pads might be grinding because of that, so in that case you should change the rotors, have them machined, or better yet, take your car to a mechanic.
- The calipers were not positioned correctly while changing the pads – In this case the caliper can drag itself on the rotor causing a grinding noise and damaging itself. In this case you should remove your wheels (first the front, then the back) and feel the entire brake assembly to see if anything feels loose. If it feels solid, spin the rotor (with the wheel still off) and listen to it. If there isn’t any noise, repeat that with the remaining wheels to track down the caliper at fault.
- The shims are not lubed enough – If there’s not enough lube between the shim and the caliper, the brake will make a whine or a squeak sound (maybe sometimes even a grinding sound). You can resolve this matter with applying a bit of silicone lube between the shim and the caliper.
- The rotors are worn – If you install new pads on worn rotors they will make a grinding noise because the new pads need a smooth surface to rub against or otherwise it will come to shortening the life of your brake pads. This is resolved with buying new rotors.
If your brakes are grinding when they are not newly installed you possibly need to change your pads. The sound you are hearing is the brake pad indicator and is normal, but you should look into replacing your pads soon.
Overall the best thing to do when your pads are grinding is to take your vehicle to the mechanic. You can fix them yourself but the extra quid you’ll pay at the mechanic are a guarantee that your brakes will have a longer lasting life.