I had the "siren problem" on my 617 turbo and the issue turned out to be a poorly seated gasket where the turbo air intake tube mates with the air cleaner housing. A simple pull it off, reseat the gasket and reinstall cured my suspected "failed turbo" problem. One of the gaskets is hard to align properly (see the Dieselgiant write up) and using some kind of jelly lube to help it seat is a good idea.

Indie was all over me to buy a new turbo.......