Tough one this.

Did the noise just appear over night or what?

Did you start the bike without the plastic clutch cover on? Noise still there?

You could try looking at the pressure plate bearing. I've seen several that have seized, but I can't say that I've come across this shrieking noise you're on about.

Does the gear shafts turn easily by hand?

Can you see any obvious signs of any components touching the case etc?

Need to be there.