Troubleshooting print issues when trying to slow down. Trying to confirm a theory (vibrations)

So I have recently started seeing an issue when printing, which has not been an issue before this weekend. This started in the space of days, not weeks or months, and I had successfully run a few hundred hours on it in the last few weeks. When I had a print that had been successfully printed fail twice in a row, I decided to check some basics. I switched to a basic snapmaker white PLA, removed all custom profiles from Luban, and printed some 3dbenchys using the out of the box config, as a baseline to try to recalibrate. What I got out was not what I expected. If I am printing in default “Fast Print” settings, print surface seems fine. When I switch to default “Normal Print” I see what appears to be a few underextruded layers and extrusion oddities in the upper hull and cabin. Not great, but gotta start somewhere. When I print in “Smooth Surface” I am now getting a textured surface, though the extrusion issue is gone. I have started getting what seems to be low speed harmonics, its growling at low speed. I am currently running off a benchy at 90deg rotation, to see if the pattern moves on the hull. I am thinking that one of my y rails maybe worked loose? I dont think its wear (I have put maybe 4-5KG of filament total through this thing since I got it), but who knows. Did the obvious stuff, recalibrated, cold pull cleaning of the nozzle, etc. My next two steps are to disassemble and reassemble the rails to see if anything is loose or if I can stop the harmonic. May potentially swap out the hot end to see what effect that has. Anyone else seen this before? Am I missing something obvious?

Here is what all three look like

Every machine has a speed at which a harmonic frequency will cause a vibration “imbalance” and negatively affect the linear rails or the machine as a whole. In this regard, each machine can be slightly different from any other machine. Usually, adjusting the print speed up or down by 3-5mm/s will fix the issue, and then you just have to note what speed it is that your machine doesn’t like.

Thats fair, but this came up in a matter of days, and has not been an issue for over a year, so something has changed. Trying to make sure I am covering the correct bases.

Hey, you should try a other slicer software to verify that its the machine or software.

The last version of Luban has many problems so i would suggest to switch to a older version or use cura, prusa slicer for example.

There could be a loose screw or something but checking software is easier ;-).