I wonder if anyone has had any experience of this?

I have been contacted this weekend by a customer that we installed a 5kw Fujitsu WM inverter for last summer.
His exact description is a strange high pitched whistling noise coming from the outdoor unit both when the system is running and when off. The system itself is working fine ,just seems to be producing this strange noise. I am going to pop in on wednesday to investigate but wondered if any of you had come accross this before??

Many thanks

Gareth