Slight layer shifting around the same Z-level  

Slight layer shifting around the same Z-level

I recently put together an i3 MK3S kit and have been printing with it for a few days. However, I've had some problems getting high quality prints and one particularly persistent problem, at least on my Benchy prints from the provided SD card.

When the print gets to a little under a centimeter above the bed, the layers shift slightly toward the front of the printer in the Y-axis, then return to true after a few layers (usually one or two). After printing my first Benchy, which displayed this problem, I printed the whistle, which did not have the same problem

I then went on to print a few other objects, including an attempt at a headphone stand, which failed badly because the Y-carriage bearings came loose and slid in their recesses while printing the infill.

After tightening the U-bolts on the bearings, I recalibrated the printer and tried printing another Benchy, which came out mostly fine, but still had the layer shift around the same Z-level.

You can see the problem is a bit more pronounced on the second Benchy. There are a few more layers shifted, but mostly around the same Z-level. Since the problem seemed to be happening around the same Z-level, I started to suspect the file itself and tried slicing a Benchy myself using the default 0.15mm layer height and Prusament PLA presets in PrusaSlicer. This one was oriented horizontally, with the bow of the ship facing directly to the right. (The provided SD card's Benchy is oriented diagonally toward the front right corner of the bed).

This one didn't come out too well. Seeing all the inconsistency in the hull made me suspect my belt tension was off, so I ran the belt test in the calibration menu:

I believe these numbers are within the expected range. I then moved the printer from its original position on top of my refrigerator, where I noticed it was shaking a bit when doing fast, jerky movements such as infill printing, to my desk, which is a bit more solid. I then fiddled with the Y-carriage bearings a little bit to check that they weren't sliding, and tried printing another Benchy off the provided SD card.

As you can see, the band of shifted layers is quite a bit thinner. It also seems to be less shifted in the Y-axis, as I could not remove it with my fingernail, unlike the shift on the first Benchy. However, it's still there. I've also noticed a slight drop in print quality near the back of the Benchy:

If you look at the bottom left corner, you can see the layers aren't closing up quite as well, and the text on the back of the boat is less readable. It seems like the layers aren't lining up as well as before.

Overall, I'd like some advice on next steps for troubleshooting the printer. I wonder if perhaps I over-tightened the U-bolts on the bearings while re-securing them to the carriage, but I'm not sure how to test this. There is some noise that occurs while moving the axes - what sounds like metallic vibration comes from all three axes in certain sections of their travel. I suspect the noise is from the bearings, but I can't confirm that. The Y-axis motor also seems to be louder than the X and Z axis motors while moving quickly, but all three axes move almost silently at low speeds.

This is my first 3D printer, and I'm not the most handy person, but it's been a pretty nice experience overall. Hoping to get these last issues ironed out so I can get to printing more useful objects. Thanks in advance for your help!