I know I don't know your machine but I slept in a Holiday Inn last night. No really I was a Sonarman for the past 20 years. It involves sound propagation, gear ratios, electro mechanical relationships as well as electronics. That is why I've found this topic so interesting. It probably took me the past 20 years to learn this; sound travels through anything for long distances and fast
. A ship can displace over 90,000 ton of water, a lot of steel. You can go to the bow of the ship and hit it with a hammer and hear it at the stern. If you place your ear agains any part of the ship on the upper decks you can hear the engines 6 decks below. Since you discarded the possibility of mechanical noise vibrating your gantry the first thing I thought of was your motors. I thought if you adjusted your tune level under this premiss; my truck seems to run better with a load, if I have something heavy in the bed it rides smoother as well, so with more torque on the engine and transmission there seems to be a better response. I would consider stepping up your max velocity is if putting a load in my truck. I understand that your steppers are running smooth but well it run better at a different velocity. You could have a week capacitor, a tired resister, a crappy power supply ……... Considering this is not occurring with every machine like yours there could be a problem with the manufacturing process. This could be originating from a parts supply or assembly. Of course this is all speculation I'm just saying some minor adjustments could give you the tell you need.
