Sounds like it might be a ground issue... To test this theory, temporarily run your Mac Book Pro on its batteries and see if the noise disappears. Not that you want to run you laptop full time on batteries but this is a good quick test to trace the issue's cause to grounding.

Ground issues are usually never just one item by itself but a combination of items interconnected but separately connected to the electrical power source. Depending on where in the world you are different solutions are available.

Thank you for your reply. Unfortunately the humming is still here (somehow quieter) when the Mac is running on its batteries.

The noise still goes on when the Montage is turned off. It only stops when the Montage USB cable is disconnected from the Mac.

Could this be a USB interface problem with the MacBook Pro Thunderbolt 3 port (USB-C) - although I'm using the right Apple USB adapter ?

I'm located in Europe (France).

I have connected my Montage to an older MacBook Pro (2012) via one of its USB-A ports, and it works perfectly with no noise or humming, whether the Mac is running on its batteries or on its power adapter.

It seems the problem could be the Apple USB-A to USB-C adapter or possibly the way the Montage deals with the Mac new Thunderbolt 3 port.

No doubt. I will make inquiries... I have no experience with Thunderbolt 3

Hello,

Noise issue greatly improved by changing the proprietary Apple USB-A to USB-C adapter for a cheaper one piece adapter (with no cord between USB connectors)!

However I had to considerably reduce the MacBook Pro audio input gain on my mixing board to completely get rid of the humming (whereas this wasn't an issue with the Mac by itself before connecting the Montage via USB).

I still believe there must be some compatibility issues to be dealt with regarding the new Mac Thunderbolt 3 ports.

Thank you again for your support.

However I had to considerably reduce the MacBook Pro audio input gain on my mixing board to completely get rid of the humming (whereas this wasn't an issue with the Mac by itself before connecting the Montage via USB).

I'm sorry, what is this audio connection between a MacBookPro and a mixing board... before we go on perhaps you want to tell us about what all is connected in your setup (and how).... since ground issues concern ALL interconnected devices.

Please say you are not using the headphone output on the MacBook Pro...

My MacBook Pro is connected to an analogic Yamaha RM800 mixer via a Focusrite Scarlett 2i USB audio interface.
The L/R jack line outputs of the Focusrite are connected to 2 channels of the mixer panned L/R.

When you said, "However I had to considerably reduce the MacBook Pro audio input gain on my mixing board to completely get rid of the humming (whereas this wasn't an issue with the Mac by itself before connecting the Montage via USB)." This tells us we really can't troubleshoot your issue this way (from this distance)... any connection can cause a certain amount of noise in an audio system. Whatever that noise level is will be assigned a value of 1, the usable signal simply needs to be a significant value (expressed in dB) above that noise floor. This is the signal to noise ratio... in any system there is noise, the key is to minimize that noise so that it is "negligible" compared to the desired usable signal.

First we need to determine what is the cause. It may or may not be ground issue, it could be many things... does the nature of the noise change when the computer is "thinking"... in other words, if you give the computer a search task can you hear it's hard drive changing the character of the noise?

There is always some kind of noise...
In other words, in any system if you look for it, you will find the noise floor, how significant is that noise compared the musical signal? A hum is meaningless if it is 100dB down from the music signal.

The audio path for your Montage is analog Out from the Main L&R to the Focusrite, the Focusrite feeds the mixer via its main analog Out, and it feeds the computer via USB.

You are not sending any audio via USB from the Montage to the computer.
If you have a connection between Montage and your computer using a USB cable, it is MIDI only.
Are you recording MIDI from the Montage to your computer?

The problem has been solved. My Montage was directly connected from its L/R outputs to its own stereo channels on the mixing board.
The Mac was separately connected to its own stereo channels on the mixer via the USB audio interface.

When I'm not playing the Montage by itself I use it as a MIDI controller with its local control off to play various VST plugins that are hosted by Logic Pro.
At one point Logic Pro prompted me to use the Montage USB connection and the Mac is now sending its audio to the Montage L/R outputs using the USB.
Humming has completely disappeared.

The USB audio interface is not used anymore in this set up. It could have been the cause of the noise. And yes, the noise was modulating along as "data traffic" could be heard.

Thanks again.