@zetof I think you are right... I've bought a sound isolator to try...
Thanks

@IntegRa
if it can helsp, please find here below my specific XU4 setting to have XU4 fan running all the time according a smooth "curve".
May be possible to adjuted depending on your fan builtin quality.
Settingof S02fan :
0_1574527848120_2019-11-23-FAN-XU4.PNG

Graph :
0_1574527869579_2019-11-23-FAN-XU4-GP.PNG

@spyhunter10 Do a clean install of the latest version of Recalbox and try again:
https://archive.recalbox.com/

Did redoing the installation solve the problem? If not, what hardware are you using? Are you using the correct BIOS? Is ROMSET correct? Have you tried changing the emulator / emulation core?

To avoid problems with Arcade games, read:
https://github.com/recalbox/recalbox-os/wiki/Easy-Arcade-on-Recalbox-(EN)
https://github.com/recalbox/recalbox-os/wiki/Advanced-Arcade-on-Recalbox-(EN)
https://github.com/recalbox/recalbox-os/wiki/Check-your-roms-version-with-clrmamepro-(EN)
https://github.com/recalbox/recalbox-os/wiki/Add-system-bios-(EN)