I have found a few things that can cause this issue. If the anti rattle springs/ shims are worn or rusted the brake pad can shift forward when the brake is applied. Worn or loose caliper slides can allow the caliper to rock when the pad squeeze the rotor.
Attention! sounds like a caliper got loose.. check its two attaching bolts. They should be torqued to about 40-50Nm, roughly half of force you use to tighten the wheel bolts.
And if they are not rusty, don't use grease, factory even uses loctite.