Archived high pitched sound while under boost all of a sudden

[DieselLady]

Try a half inch wrench on the rear exhaust manifold bolts. It is common for them to get just a little loose and you may not know it till you get a little drive pressure one day and it shows itself. I put an e brake on a friends truck and that is how we found hers. Sounds like a gasket vibrating between the two surfaces? It can go away with heat which might explain the noise stopping at a higher boost.
If it sound like it is coming from the turbo side, I would bet on this.