Police siren noise

[shell74]

Hi everyone. hoping to get some insight on the problems we're having on our poor little passat estate.
It's a 1.9tdi, 2002 B5.
Over the last 2 weeks it's had a police siren noise coming from under the bonnet. Been asking mechanics that are friends at shops, garages, and forums as to what it could be. No one seems to have a clue. We've been told it maybe that there's a split pipe from the turbo, could be the intercooler or the turbo itself. My fella took the intercooler pipes off this morning and found oil in them. Quite a amount as well, but no splits anywhere.
We haven't lost any power from the turbo and this is why it's puzzling, we really don't want to be spending a fortune on a new one just to find out its something else.
We also had it put on a diagnostic machine yesterday and there was no faults found.
Please could someone help as to what it's likely to be.