Hi Gumbyger, thanks for the response.
It is difficult to describe a sound by words alone. The noise is a constant throbbing note, louder than all the other sounds that you would normally associate with a piston engine i.e. pistons' ignition, exhaust, propeller etc. It's there once the aircraft is at normal flying speed, but much quieter when on the ground. The nearest I can describe it is similar to the sound when riding on the London Underground! It seems to be louder in my right ear- if that's any help
Sorry, I can not be more precise. If that is how the engine and aircraft sound in real life, then fair enough. But I find I have to keep the speaker volume at a low setting in order to tolerate it. Very uncomfortable, for me anyway.