I had a local mechanic go over the entire car and he wasn't able to determine what it is. He check the sway bar bushing, ball joint, engine mount, and chassis.
The noise only happens when you step on the brakes with a good amount of force, it won't happened if you brake softly. It also only happens at low speeds. At high speed you either don't notice it but it is not an issue. It happens only going forward braking to a stop but not backward to a stop.
I remove the caliper to take a look inside the piston there was a lot of rust build up. I removed a lot of it, but the sound is still present.
I have a feeling it might be a caliper piston issue? It might be sticking or has too much play. It sounds like a vibrating knock sound. You can hear it and feel it through the steering wheel.
