300tdi strange air leaking noise

Well, yesterday I took off the PTO cover on the transfer box to look for signs of wear on the input gear of the T/B and on the output shaft of the gear box. They look brand new, and it is the upgraded cross drilled version. The bearings also seems in very good condition.

I have tried slipping the transfer box into neutral a few more times and the noise is definitely associated with it. Is there any other checks that I could perform to check the other internals other that removing and overhauling?

Drain the oil and take the sump cover off?

The handbrake drum and some of the bottom end of the transfer box will still be turning when it's in neutral and the car is rolling so that's not where it's coming from. The other thing that happens when you coast in transfer neutral is that the main gearbox output stops turning. Check the oil in both boxes.

Hard to think of what to do next - apart from getting the garage to take the transfer box off and having it rebuilt. You have checked the oils, and all the obvious things. As said above the output bearings/handbrake parts are still revolving when the noise disappears in neutral, although the bearings are not under load. It might just be worth removing one propshaft, putting it in diff lock and repeating the test, in case one of the output bearings is noisy under load.

Forbes said:

Faircot! Genius! I finally got round to testing the transfer box as you described tonight. Guess what!? No wooshing, hissing sound when the transfer box is in neutral! So the question is now... What's wrong with it? Transfer boxes sound expensive....

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I must admit I took the easy way out and shoved some Molyslip into the transfer box when I changed the oil and it certainly quietened it down once the oil warmed up. It's been like that for a few thousand miles now with no noticeable deterioration but YMMV.

Just a long shot. I picked up your comments about "down on power etc" and I'm wondering if the inlet/exhaust manifold gasket is leaking and allowing an escape of air. I changed mine on my 300 TDI because it was one of the places where I had oil leakage but no noticeable noise. I can't say I noticed the engine being down on power prior to changing the gasket but I can definitely say that since doing the job it is up on power and more torquey, and whilst the engine is fairly noisy, it is smoother with a different note. Might be worth a look.