My SDL has a high pitch squeel or a whistle when the boost goes high (starting at around 2500 rpms or so, depending on the grade).

Took it to have emissions test yesterday and the tech thought that it was coming from under the intake manifold but it's hard to locate the exact spot. I've tightened one of the bolts that I found loose and the noise diminished some but it's still singing (just not as loud).

I have replaced the intake gasket when I took off the manifold some time ago. Is there any compound that one needs to apply when re-installing the gasket?! The gasket was German-made (Viktor Reintz) purchased here.

Has anyone experienced that kind of condition and do you have any suggestions on how to narrow down the search!?

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K