My Longmill has been working well but I was always getting a bit of whining from what I think was the right Y rail/wheel when I jogged the machine from front to back. A few weeks ago I removed the screws in the feet and ran it back and forth to try and improve parallel with the other axis. I screwed it back down and did notice an improvement but I still had a particular spot where it was whining quite a bit, especially a jog speeds over 3000mm/min.
Over the weekend, when playing with my homing sensors, I was looking closely at both Y axis. After a homing cycle I noticed the left gantry plate was in full contact with the rear foot but the right one was maybe 1mm off the foot, indicating I was very slightly out of parallel left to right. I corrected the issue and noticed the machine is much quieter when jogging all the way forward and back now. Still a spot where it whines a bit but definitely an improvement.
I don’t know how long that was out, I believed I had checked it before during setup. It was such a small variation I almost didn’t notice it, but I did notice the improvement when it was corrected. Hopefully that helps the next person.