main dealer said NO fault found? less than a day later it's still got the fault! 

I cant tell if the sound is coming from a compressor mounted deep in the engine or from that squirrel cage fan box?? I presume the system is running as a heat pump ( reverse cycle ) to pump heat into the batteries or cabin. If so, this sounds like a dry compressor ( no oil, has there been an issue with refrigerant leaks), or the reverse 4 way valve, is not closing off fully, like having a faulty check valve, or is bypassing and cycling super heated refigerant vapour accross the 4 way valve, or bypass valve. This wouldnt show any fault on the ecu, as the 4 way valve only shows electrical working or not, only way to know if it is working is to have a set of refrigerant gauges connected when this happens. Does the garage have access to that kind of equiptment and ability to diagnose?? May be better off with a garage that deals with air con leaks etc. Good luck.

Hello ! This sounds very much like the HVAC compressor has a mechanical issue which wont show up on the fancy Renault Diagnostics. And as someone said before if there are no fault codes it must be fine - yeah . Sound like the scroll has either smashed itself to pieces or at least a chunk has came off.