how can i fix my turbo vauxhall vectra, isuzu 1.7turbo diesel?

how can i fix a problem on my turbo on my vauxhall vectra 1.7 turbo diesel isuzu engine, the turbo does work but it whistle's and it doesnt kick in to the engine (because the car is slow), ive checked all pipes for splits (cannot see any), ive replaced the the whole turbo, and still no joy,its only been happening since my headgasket was done by a mobile mercanic, can you please help me thanks..