Yaesu FT-991A spectrum-scope / waterfall display noise through headphones work-around

After the work-around for the "no PTT in cw mode" bug. I started to think I might be able to use the FT-991a in a cw contest. On plugging in my headphones to the phones socket and listening through them, I noticed a background noise that sounded like a whine with some data like noise inter-mixed. I changed headphones but the noise was always present (more noticeable with low volume). For some reason I pressed the function key and noticed the noise disappeared (same effect with pressing the menu key). I soon realised that the noise was only present whilst the spectrum-scope / waterfall was visible and actively updating. I had a think about making up an audio low pass filter to go in series with the headphones but then thought better of it in case it let loose the magic smoke on such an expensive radio. I then started to play with all the scope settings like dimmer and colour to see if the whine would go away. Eventually I hit the solution, the noise would vanish completely when the "moni" (monitor, in the function menu) value was reduced below 20. Hopefully the time I spent finding this work-around will help someone out there!