Noticed in the last week that when braking at speeds below 55, I get a single clunk or a knock on my STi. Definitely feeling it in the front. I recently replaced the rotors with Centric High Premiums in July. I pulled the car up on ramps last night and checked all my bolts that I removed to install the rotors and they were tight. I gave them a jiggle test, did not touch anything with a socket. Aside from noticing a recently slung open inner passenger side cv boot, I am out of ideas. I did have a problem when taking the old rotors off that they were seized to the hub. Speaking to a mechanic friend, he said his STi did this when he got some rust on the hub.

I would check the tophat mounting nut for the strut.

The backs of the brake pads occasionally need a little grease. Without it, they will sometimes clunk.

Yeah, that is a good point to start with. When I changed my rotors, I left the pads in the calipers so I didn't have to fool with all the pins and clips on the brembo 4 pots. Just picked it up and sat it out of the way as one whole unit. May just need to get some lube in there. Will try it out whenever I get a chance.