Grinding/ groaning from front wheel

I have a 2012 with 170k miles. Recently, when resuming forward movement after a stop, the left wheel would occasionally emit a quick sort of groaning sound, as if the brakes were reluctant to let go of the rotor. I could actually feel it. Once the car was moving, all was fine.
Wondering if the brakes were binding due to slide pins in need of lubrication, I removed the wheel. Though the pads had a lot of life left in them, I did notice slightly more wear on the inside pad. The slide pins were somewhat stuck in place but, once spun, they freed up quite a bit, bouncing back after pressed in by hand. I stupidly didn't lubricate them, as I had nothing to use for this and it was more of a fact finding mission. I threw the wheel back on and intended to get down to lubricating all locations soon. A few days afterwards, for the last 4 to 5 days, the same wheel has displayed some new symptoms. When in motion, there's an intermittent grinding or rattling sound. It got pretty bad when on an out of town trip yesterday. The symptoms were most notable on surface streets. And they came and went. Sometimes it seemed to increase with rough road surfaces. It got so bad that I thought something spectacular would happen, like the wheel coming off entirely. So, I cut the trip short and headed back home, 200 miles. On the way home, the issue cleared up, almost entirely. When back in town, I drove about a mile around the house on surface streets, and couldn't replicate the symptoms, apart from a very slight and short variation of the grabbing sound after resuming from a stop. This is 200 miles after a catastrophic failure seemed eminent!
It made me think of a bad bearing, though I've never dealt with a car's wheel bearing, specifically. These sounds don't seem to correlate to the sounds ascribed to a bad wheel bearing. There is no whining, scraping or other sounds changing with speed like has been attributed to a wheel bearing. Maybe it's a suspension issue. I don't know how it can be so intermittent.

 

I lubed the pins on both front wheel calipers. The one in question had an oversized bushing on the lower pin that had escaped the groove in which it's supposed to sit, then traveled up the sliding pin an inch or two. I'm thinking that someone used regular synthetic caliper grease to lube the pins instead of Sil glyde, causing the bushing to swell.

The wheel is still displaying the same issue as before. It's as if the caliper piston doesn't always want to immediately retract when the brakes are disengaged. And it seems as though once the piston does decide to retract, it doesn't do so fully, causing the brakes to strike the rotor with a randomly repeated, um, bumping. This only intermittently happens when the car is travelling at low speeds, sometimes when I'm using the brakes, other times when I'm not touching them. It can happen while going over speed bumps, those big humps in Los Angeles, when I'm landing on the far side of the hump without braking.

I don't know if I have it right and the tight tolerances are causing this. Or if there is something causing the piston to actually push out out at inappropriate moments. Maybe the ABS is malfunctioning and engaging on one wheel only. I really know nothing about the ABS.

My gut says that the caliper piston is simply failing or in need of lubrication of some sort.

Any ideas would be appreciated.

Thanks

 

At least on the front, the O-rings are on the lower pins. I neglected to say that I replaced the O-rings when I lubed the pins. I did not replace the boots. Nor did I clean the pin cavities. But with the generous amount of Sil glyde that I used, I would expect the brakes to behave somewhat differently than before. But they are acting exactly the same.

 

Long story short. I tried tracking down this issue for months, unsuccessfully. Then one day I replicated one of the sounds by simply exiting the car. It groaned when I relieved it of my 290 lbs. It was a strut, LOL. Had both replaced in front and all is right in the world.