Could Possibly be the infamous manifold squeal sound a bit like a slipping fan belt.
Check the exhaust manfold for sign of leak and missing nuts.
The exhaust manifold on TD5 warp braking or riping out the studs in the head Either on cylinders 1 or 5.
Solution you need to take Exhaust manifold of have it machined flat drill out the holes where studs have broken to give a little more clearance, You will need to replace studs which is the real problem depends on where they have broken or whether they have torn out of the head which will need helicoiling before you fit new stud.
Have a similar problem with mine and had replace the belt, Most of the pulleys, the steering pump, water pump, and turbo before I was told about this issue.