Howdy
So i was sitting on the crapper reading hot 4's when i found this. I thought, mmmmm, maybe this could help you. Apparently these gaskets can cause the sound you hear, maybe check it out. Sorry, this machine refuses to upload the article. Basically, the metal gasket between turbo and exhaust sometimes breaks up or disintegrates coz of agel. It then makes funny sounds at the turbo, like gas escaping at high pressure, or gentle knocking sound. Check yours out, may help you.
I am now going to upload my fist into the computers A-drive and see how it likes it!
Good day to you.