Will be a belt slipping, often happens in wet/damp weather. Check the tension of both ancillary belts, the belt should deflect about 12-15 mm in the centre of it’s longest stretch
Got to the bottom of my squealing noise, after a few hours in the garage ( also getting new springs on ) turns out there is something leaking onto my fan belt… can’t remember the name of the part, it sits just above the belt and its the dripping of the fluid on the belt that causes the squealing noise, whatever the parts callled it has a t-junction on it and this is what needs fixing, and of course it can’t be bought seperatley so its a whole thing to replace. Can’t use the car until this gets fixed for fear of blowing the head gasket or something like that…the garage recommended this…
at least I now know what the problem is, but I have to say, I didn’t realise that not being able to drive my Mx5 even for a few days would annoy me so much…