Is It The Turbo

I have 2009 T300 with a Paccar(junk) PX-8 engine. It developed a whistling noise as if you have your window cracked a little.Only after 35mph.After driving it for sometime I narrowed it down to it was mostly making noise when the engine brake was activated.If I cut the engine brake off as the noise was occurring it would stop.I then operated the engine at it's maxiumnd RPM it could hear the same noise but faintly. I then had someone drive while I listened from the passenger seat. The noise was obviously coming from the air scoop on the side of the hood.I did remove the turbo hose to check the finns and bearing play the appear OK. But did not check the back half of the turbo.Im thinking the turbo is going. Am headed in the right direction? Any help appreiciated and THANKS IN ADVANCE