Grinding/Whistling Noise

I have a '91 Twin Turbo Automatic, and when I go above 3k RPM, I get a grinding noise for a short time, and a huge power loss. It does eventually pick up, but I get a constant whistling noise, but not all the time.

Sometimes when I do put my foot down, it just drops into the right gear and it shoots off like a rocket, but 90% of the time it will delay massively.

A few different people said it is a gearbox issue, but someone who has had more experience with turbo performance cars says it's a turbo. The previous owner said it was like that for over a year, and it is still going strong, so I'm not 100% sure it is the turbo.

I'll try to make a vid of it if the weather improves at all, as I don't like putting my foot down in the wet.