I didn't have a lot of time to investigate the issue but I needed to get on with stuff so I swapped to another i7 machine running Linux Mint. Sound was good from the new machine using the same audio power amp but when I tried to install Zoom it went through the motions but it doesn't function, at all. Another issue to chase up!
Yesterday, I changed back to the old i7 machine running Linux Mint where Zoom does work but the sound was bad. I noticed that the wooshing and buzzing background noise was modulated by the movement of repositioning the machine under my bench. Further investigation shows that there are some screw heads slightly proud on the bottom of the amplifier which sits atop the computer box which have rubbed through the paint on the top of the box (amp has rubber feet but they are spaced wider than the top of the computer case) and are providing some sort of earth connection between the computer and the amp. The computer is powered from the mains with a 3 pin mains lead and has a solid earth to its case whereas the power amp is on a two core mains lead and must normally float with respect to mains earth. Obviously, there is an earth associated with the signal lead and the amp is grounded sufficiently to stop audible mains hum in normal use. It seems the the computer is not happy when its audio ground gets wired back to its case via the power amp chassis. Sliding a sheet of cardboard between the top of the computer and the base of the amp has solved the problem.
All is not sweetness and light, however. During a Zoom meeting last evening the machine lost much of the display, some windows on the screen stopped moving, some went black and only the sound carried on operating. The mouse pointer was still alive but nothing could be "clicked" and the keyboard was dead. The only influence I could have was by pressing the reset button. After rebooting it was all working as per normal for about 20 minutes and then it locked up again. There is something odd about Zoom and Linux Mint. All day yesterday up to the Zoom meeting and all day today the machine has been fine. I have been watching the steam trains operating from Grosmont on the NYMR, some Youtube videos and some BBC news videos and everything has been fine, no lock ups, no bad sound.
Thank you to all of those who made helpful suggestions regarding the noises issue, at least it was a no cost solution when we got there.
Rans6...................