I agree with Herman, Nemrod and Simonbeans above. Try it with a different tank, or with that tank with the J-valve down. If it still squeals, its the regulator. If not, it was the J-valve. I have a Scubapro J-valve that squeals like that when up.
If it is the regulator, it is probably a Mistral. I say that because this noise is sometimes a characteristic of that regulator. It is possible there is some corrosion in the venturi nozzle. It can be unscrewed and put in an acid solution (white vinegar for a minute or so, or a regulator acid solution). If the noise persists, you can play around with the orientation of the two side holes in the venturi nozzle. I have played around with this a bit, and found that I could put teflon tape around that nozzle's threads, then because it is tighter in the threads I could orient the nozzle so that the holes are parallel with the bottom box and the lever platform. This will re-direct air around the inside of the case, rather than having one oriented upwards towards the lever platform and the diaphragm. But all this implies that you are comfortable opening the boxes and looking inside, and from your posts I'm not sure you have that knowledge.
SeaRat