Concerning noise after spark plug replacement

Hello all, a few days ago my g2 Prius maintenance light came on for an oil change. As the oil hadn't been changed since February or so of 2019, I went ahead and did a change. All was well, no leaks, engine was running fine. I went on a shakedown drive after checking the oil and cleaning up, and noticed that the engine was acting a bit strange. I wasn't quite sure what was going on, but it felt and kind of sounded like misfire. It only seemed to occur at idle and low throttle, but gunning it or mid-high throttle was 100% normal.

Considering my mileage (almost 99k), I decided to go ahead and replace the spark plugs. Assuming the noise was misfire, it would fix that alongside being good maintenance. When I went to remove the old spark plugs however, I encountered a problem. The plug on the second cylinder (second from the left) was exceptionally tight and I had some difficulty getting it loose. Once loose, I tried to extract it, only to realize that the ceramic shell had shattered and numerous ceramic shards were kept out of the cylinder by the spark plug. I used a vacuum and air compressor to extract the shards I could see, and then managed to remove the broken plug. The other two plugs removed with some difficulty, but did not break like the second one. I put in brand new NGK Iridium plugs, put things back together and went to fire it up. Turns out, a piece of ceramic off the broken plug was missed and fell into the cylinder; the engine rattled and shaked pretty bad at first, but then the noise stopped and the engine returned to normal operation. Or so I thought. After some shakedown drives, I noticed that one of the pistons (presumably number two) was "slapping." It's noticeable at engine idle and low revs, less noticeable at mid-to high throttle but you can hear it if you listen carefully.

After researching the spark plug thing, it sounds like there could be some damage that could vary from minor to major. I did not gap my new plugs, so I wonder if its possible that is the culprit? However, my larger concerns are piston and/or cylinder wall damage that could cause serious issues or reduce the engine's life. Seems like it could also be a bent valve from the piston slapping it when the ceramic piece was lodged in the valve when it opened. If the head isn't too difficult to remove, without removing the engine, then I would likely want to try and remove it to take a peek inside the cylinder to check for damage to the cylinder wall; nicks on the piston itself aren't of major concern to me. I would think I might also be able to check for a bent valve if I were to remove the head as well; and assuming everything is fine, replacing the head gasket would be a nice peace of mind as my mileage creeps up over 100k. If removing the head is not easy, then I am considering getting a bore-scope to peek at the cylinder walls/piston head; but I don't think this would let me check the valves.

I can say that what I thought was misfire is gone with the new spark plugs, so I do think that was my issue. However I may now have a more serious issue than before. If you have any other suggestions or advice in regards to determining potential damage or fixing this, do keep in mind that I am fairly new to car work and do not have more than basic tools; I also don't have a lot of funds for tools and things, but I am willing to buy parts that I can install myself. I have a second car that I can use in the meantime, but it can be somewhat temperamental and isn't exactly reliable; so I would like to deal with is quickly and get my Prius back to it's daily driver status. Sorry for the text wall, just thought I'd provide as much info as I can!

PS: I do have video/audio of the misfire and of the "slapping" if you need me to post links. In the misfire clip, the slap is not audible, and I have not heard it before when driving.

 

My current game plan is to take a borescope and inspect each cylinder to confirm if there is any internal damage or not. I'm not necessarily opposed to replacing the engine, but that seems quite expensive considering my current funds as a college student. Majority of my spare cash was supposed to go into my other car, lol.

I don't know enough about piston engines to really be able to diagnose this kind of thing.
If I were to go the motor replacement route, I'm not really sure where to even begin. And if I were to acquire a motor for a good price and decent mileage, I don't have the tools or help to do it myself, and I do not know if I can trust many shops to install the new motor correctly. I have heard in the past that not many mechanics know how to work on these cars....

 

Update:
I was recommended to try adding Seafoam to my crank case to see if maybe the noise is actually lifter tap or something in the valvetrain. I added some, but not much. It seemed to help a bit. However, I checked my oil after a short drive around the neighborhood and it appears I somehow may have added too much oil during my oil change (despite the fact I measured it out quart by quart and even spilled a little bit). I'm wondering if having too much (nearly a half inch or more over the full line) is causing the noise. Definitely going to drain some oil and see where that gets us; I'm also contemplating going all the way to the Toyota dealer to get an OEM filter in case its the Fram filter that's causing an issue.

 

I've got another update:
I changed out the Fram oil filter I put on during the oil change and replaced it with an OEM Toyota filter. My oil level has dropped back down to normal. However, I can't really say if the noise has gotten better nor not; though I can say it is not worse.

As I said before, I don't know much about piston engines to actually diagnose this. I spent some time on YouTube watching various videos of different noises: knock, piston slap, and rod knock most specifically. But I cannot really tell if this noise is any of those.

This is the noise I'm hearing, you'll have to listen over the tick of the injectors.

I have an alternative video where the noise is a bit more noticeable at the end.

The noise heard in this video seems close to what I am hearing, but I don't really know.

I appreciate your suggestions and recommendations so far.

 

Alright, its been a couple months since the noise appeared. I had the car diagnosed by a local shop (they did it for free) back in January, shortly after I made this thread. They stuck a camera into the cylinder and confirmed the presence of a couple small porcelain bits that must have slipped past the spark plug at some point during my attempt to extract it before realizing the porcelain had shattered. They told me that they attempted to remove these pieces by sticking adhesive to their camera, but the pieces appeared to be stuck. They also told me that there was no scoring on the cylinder walls and that there was little risk for any long-term damage.

Besides the piston slap-esc rattle, the engine runs fine and still makes power, so I'm just letting be for now. The long-term plan will probably be to have the head gasket changed early so these pieces can be removed easily, but only time will tell.

I appreciate the help and suggestions!