MMI intermittent interference when using audio/carplay

[Simon M]

An on and off problem, some journeys fine, others shocking audio. I would say 50/50. 
Essentially, when using either Bluetooth or cable to USB connection the sound quality has what I can only describe as sounding like bust speakers, high frequency noise all over voice (telephone) or any audio, radio, CarPlay even aux like an iPod. 
Drives me crazy then next time it’s totally fine, with great sound reproduction. 
I’ve tried different cables, different phones etc. and nothing changes. 
Had the A5 at an Audi specialist (not a dealer) they carried out a ‘hard reset’ saying it might be that. No improvement. 
Has anyone got/had the same problem and PLEASE can you help 

Simon