Thin steel shims behind brake pad fell out

You could try some CRC disc brake quiet gue .You put it on the back side of the pads and it quiets this kind of noise down.It's like silicon sealant that takes up that space between the piston and the pad.I use it all the time on my 76 Dodge Aspen and my 71 Chevy C20 's disc brakes
Advance Auto Parts - Down for Maintenance