Brakes sticking?

Just today I noticed my brakes "clanging" when switching from forward to reverse and vice versa. When pressing the brake pedal after moving forward/reverse, I hear a "p-clink" sound. The relation to the brake pedal makes me question what's up with my brakes. It seems to happen more often than not.

Yesterday I had my truck oil sprayed - perhaps some gunk is causing the pads to stick? Ideas please and thanks.

Edit: I have 51,000 km on the truck.

 

Bump. I did some looking around online and found a thread on TW that hinted towards needing to replace the brake pads. Like the OP in that thread, I am experiencing the "clank" only on the first brake operation after switching to forward or reverse.

What causes the clanking, if indeed the pads do need to be changed?