That sounds like it's related to the turbo, it responds to the increased engine RPM just like a turbocharger does. If it's a boost leak, you'll spot it by the tell-tale black sludge around the leak.
If there's no boost leak (check the intercooler too), pull the intake hose off the turbocharger and give the impeller a wiggle. It shouldn't move much at all - if it does, your turbocharger may be on the way out. You may not notice a reduction in boost pressure if it's only just starting to let go.
Oh, there is one more thing it might be. On mine (slightly different sound, but you'll understand why in a moment) my exhaust gasket between the turbocharger and the exhaust manifold was eaten away by the heat just enough so that it became very much like the reed in a clarinet. Mine was a rather badly played clarinet, and I wasn't asked back for a repeat performance, but replacing the gasket fixed it. Look for signs of soot around the joint.