Found the problem...it is definitely the coding for the Active Noise Cancellation system (Accessible through Phone App, three dots menu/Settings/Accessibility).
When you turn that system OFF, the problem appears to go away, but the phone does NOT save the setting when it is turned off - it reverts back to a default of Noise Reduction turned ON.
So you have to remember to turn Noise Cancellation off every time you restart the phone. Which is not a good solution.
This explains why people who turn Noise Reduction off are reporting the return of the problem later on. It comes back when you restart the phone.