2000 Mitsubishi Pajero engine whistle above 2500 rpm

Hi this is my first time out and hopeful of some help with a Mitsibushi Pajero engine problem.This vehicle has a long history of engine problems, mainly with the cooling system, which I've hopefully now sorted. This latest problem with a 'whistle' noise coming from the engine occured a week or so after I fitted a new head gasket. Initially I put the noise down to a faulty turbo, which had excessive impeller lift and was blowing out a considerable amount of engine oil. I replaced the turbo and have checked all hoses and connections from the turbo to the intrcooler and to the inlet manifold. The inlet manifold and turbo wastegate all have new gaskets fitted. Could anyone please offer some advice?