Is my turbocharger blown or is it a boost leak?

I've got an 87 Toyota Supra single turbo and this is my first turbocharged vehicle so I'm still trying to learn about them. I've been driving my car around fine up until this morning. It seems like after the car warms up the turbo will make a "whining" noise kinda like a supercharger for just a few seconds when it starts to boost. Then it stops! After I stop boosting then start again, it does the same thing! The car is still hitting 5-7psi no problem, but once I hold steady boost for a while it seems like the car is losing power.