Humming is likely ground noise. If you live in the USA Ground hum sounds a musical tone between a low Bb and B (closer to a B natural)
USB ground noise is usually described as a high zinging noise. USB ground noise will be influenced by activity on the computer... ask you computer to search for something, if the zinging noise corresponds to the computer activity, it is clearly USB ground connection noise
Ground noise is caused by the interconnection of devices and how they deal with electricity.

Interconnecting devices causes ground hum. Try disconnecting devices, while the DX7II remains connected, to see if the situation improves. If it does, then find a way to deal with ground noise.

Tracking down ground noise is best not handled on a website especially where you don’t state all the facts (voltage) ground hums is one thing, electrical grounding another. You can get bad advice when your at 220V and the advise is for 110V.

Silly things like plugging keyboards into different AC sources can cause ground issues; try to make sure all your devices are receiving AC from the same source... power strip.

Any noises other than hum, is a nature of living in the 80’s, (you should be used to that). But you can remove the ground noise with diligence.
Since the DX7IIFD predates USB, you have not described how you have it connected, other than a USB to MIDI “cable” (?)
Homemade? Or is there some conversion going on (an adapter)?

In general, when hunting down ground problems, don’t get creative, Get Help.

The soundcard of the MacBookPro...

Sorry. Yes I live in the U.S. Michigan to be exact. My concern is its fine on M1 but not on dx7. The dx7 is plugged into a different powerstrip than M1 so maybe not grounded as good. You may be on to something with the computer because simply moving my fingers on the touchpad of the macbook makes even more noise.

So the midi cable is a cheap one here: Cheap one

so I am thinkning I should go to this: focusrite

Try unplugging the Mac, run it on batteries. (Not optimum but will allow you to eliminate the USB noise).

And most external interfaces are going to be an improvement. Computer companies when they think music, think about the people who PLAYBACK, not musicians - the people who PLAY. So yes, an external soundcard (audio interface) will be an automatic improvement.

Happy holiday!

My rig is in a different room than where I normally keep my computer so I always unplug before heading in there. My battery lasts a long time.