Hopefully i can help..
First off it would be well worth by checking "all" intake / intercooler / turbo piping hose circuits..to make sure they are properly secured and none have any small tears or splits in them or restrictions causing the hissing noise your hearing,if under acceleration then this is most likly what it may be especailly so after the mass air flow meter as a leak after that can in a sense cause the black smoke as it now has unmetered air the ecu cant see going into the turbo..so starting off from the airfilter and check condition of that and make sure it not blocked up and is seated properly and follow the tubes round carefully..also check intercooler piping aswell especially - then lastly check around the turbo unit and the piping to that aswell and intake manifold aswell as not forgetting to inspect the exhaust manifold and the down pipe for any obvious signs of exhaust gas leaks there..also too check around the egr valve aswell "exhaust gas recirculation valve"..If the hissing can still be heard at idle then you should be able to pin point it when your checking around the engine and that should help narrow down the area or location the above noise is coming from or you could get somebody to increase the revs slightly and see if that can help pin pint exactly where the noise is coming from..I do hope this helps
thank you will use this to check all pipes. Now also going into limp mode and some turbo whisle
Hi..there thanks for your reply..
Yes checking those pipes etc would be the best place to start anyway aswell as any restrictions etc..failing that then the problem may lay with the turbo itself/intercooler/turbo wastegate stuck..but as above best to check pipe work first.If there are any engine management lights on during limp home mode then a diagnostic test may be something else to consider next.I do hope this helps further