Groaning/Knocking Sound When Braking

[GrimeBot5k]

Hey all!

I really need some help diagnosing this ongoing issue I've been having (I apologize in advance for the wall of words, I've been trying to take really good notes as I go, so that way it's easier to get help)

Here's the deets:

2013 with 15mm spacers and aftermarket wheels. 55k miles. Never tracked, handful of spirited drives, but none with significant/sustained heavy braking. I've owned the car since April.

Front/Passenger wheel makes a groaning (Like the sound a trampoline makes, but slower)/knocking noise when braking. I can't feel it in the brake pedal.

I first noticed a slight noise when braking, figured it was the brake pads starting to go as the car only did it when the wheels got wet. As I was taking the wheel off to replace the pads, I noticed that I had 1 missing lugnut and another that was barely on.

After getting to the pads I found that they are fine, plenty of life left. Got new lugnuts, but as I was putting the wheel back on, one of the studs came off. Replaced the wheel studs for the spacer w/ ARP studs. Got everything bolted back down and the noise went away. A day or so later the noise came back so I replaced the wheel spacer nuts and swapped the front spacers and the noise went away.

A few days later, the noise came back; this time I went about it more methodically. I jacked up the front end and noticed that the wheel did not spin freely on 2/3 sections of the rotation.

Removed the wheel, and for shits and giggles I pulled the pads out. Wheel spun freely. Put the pads in, and it would catch. Caliper piston had not been compressed while I was doing this.

Took everything off down to the hub. Caliper piston was compressed during this.
Hub spins freely, no noise whatsoever.
Put Rotor, caliper, pads back on. Put the spacer back on finger tight. When I tried to spin it I heard/felt it get stuck again. So I pushed the rotor into place and re-finger tightened. Then it spun freely.

I did note that the inside of the rotor (part facing the engine) has 2 sections that have 3-4 burn(?)/rub marks.

As I put everything back together I made sure to tighten everything to factory spec using hand tools. Even went so far as to finger tighten in star pattern. Car ran without noise for 4 days.

Noise came back today. Jacked up the front and the wheel was catching in the same way again. Torqued everything down and the wheel is spinning mostly free again.

I was leaning towards the rotor being the culprit, but now I'm thinking that the marks on it are symptoms and not the issue itself (I know they will need to be replaced regardless). After doing some research, I'm pointing towards it being something with the caliper.

Like I said before, I tried to lay this out with as much detail as possible. I'm fairly decent when it comes to wrenching, but diagnosing is not my strong suit. Any help is greatly appreciated! I'm at my wits end.

Thanks!

[strat61caster]

Sounds like the caliper is sticking. Do the slide pins rebound quickly when pushed in? If not pull them out of the caliper clean and regrease. And even if they do fuck it grease em anyway. Chuck a layer of grease on the bracket between the pads and the bracket so they slide easily as well and can retract from the disc freely while you're there.

Outside chance it's a wheel bearing but I just had this exact problem on my wife's car a few months ago. These cars are coming up on a decade old now, get ready for old car problems

[GrimeBot5k]

Huh...I'll check that tonight, but I feel like I remember one of the slide pins not really even rebounding.

Whelp. This makes more sense. Thank you for the suggestion! I'll try that either tonight or tomorrow, depending how cold it is tonight.

[GrimeBot5k]

Update: Got the pins out easily enough and the rubber boots were in good condition.

I didn't actually realize that they *weren't* supposed to be already compressed until I got them greased and reinserted. There's still the damage to the rotor I'll need to address, but this is already a marked improvement from where it was at.

Thanks again, and thanks for the tip on removing the rubber boots!

[strat61caster]

The rotor itself is probably fine and the marks can clean up with a solid brake bedding session is my guess. Hope this solves the noise!