I have an AO-100, still in original AO-100 configuration with original firmware. Recently (last night) I noticed an unusual vibration in the X and Y travel.
I have a bit of experience in CNC setupm and is seems as if it is a motor tuning issue. This causes the printer to loose its position midway through a print (~ .500" off) in one direction. The print continues fine afterwards, but when losing its steps, a fairly ominous buzz / vibration occurs (again, sounds like the stepper is fighting itself).
Additionally, the vibration seems to be stepper induced rather than an issue with the linear motion slides as it does not change with light hand pressure on the carriage or table while in motion.
The vibration is evident int eh printed material as well as the extrusions are not smooth, rather slightly rough looking.
I have the smaller motor pulleys to change the X adn Y drive ratio, perhaps this is a good time to change them out and tweak the machine parameters accordingly to see it it helps.
I have been using the machine for some parts for my turbocharged dragbike, namely brackets to relocate the fairings, as well as some tools to aid in marking tubing for the turbo system plumbing.