Our L200 Trojan makes a whistling noise when engine underloading after an exhaust back section replacement. Exhaust dealer says just to wait for the exhaust pipe to go back from soot to cure it but i'm not convinced as it was done in March and still whistling. any ideas please?

More information please. What year is this vehicle? Is this a constant high rev or is it surging high then low and back again? Are you sure it is oil from the exhaust? Using a pair of rubber gloves, rub some of this 'oil' between your finger tips, oil will feel greasy. Sometime soot can form in the exhaust, especially on diesels, and when this mixes with water it can look like oil.
This 'Air Noise'. Is it a whoosh, whistle. hiss? This information can give clues as to where the problem is. Air noises, over rev and oil from the exhaust could be more than one problem. Get a Good mechanic to look at this and to check the Turbo and all the pipes for issues.

check hoses for leaks or if they are disconnected

A whistling noise can be generated by an alternator at high RPM's and indicates a slip ring brush problem or a diode failing . It resembles a frequency whistle.. IT may also be from a leaking gasket and a partially blocked exhaust system from a failed cat converter and at speed and power, the exhaust gas is forced out the hole

Exhaust manifold leak? That's not common. You talking about the VVT near the manifold at the top of engine above the power steering pump? Yes indeed, AC Delco is the only one our engines love. 41-103. Obviously you have P0304, what other codes?

You have ruined the exhaust converters with the diesel fuel

Its that dealer next to usless?

Take it to an independent (not a chain) muffler shop and have it replaced. I'm guessing around $200. Also, have the engine mounts checked. If they're worn, the engine rocks too much and breaks the flex pipe.

Remember, what is heard as turbocharger noise may not be caused by a noisy turbocharger. Before replacing a turbocharger perform the following diagnosis.
A. If a loud whine or whistle is heard during acceleration and increases or decreases with rpm and load. This condition is most often the result of a loose clamp/hose at the turbocharger compressor outlet, charge air cooler inlet, outlet or at the intake manifold.

1. With the engine running at idle and the transmission in Park or Neutral with the parking brake set, feel for boost air escaping at each connection between the turbocharger compressor outlet and intake manifold. For limited access areas, spray soapy water on those connectors and look for bubbles.

2. Check the exhaust manifold and exhaust system for leaks.

I hope this helped.

Thank you,
Norse

locate the whistle noise with a hose with one end to your ear, then probe the exhaust connections until the noise gets loud in your ear.

If it's near the catalytic converter and performance is down, it could be plugged.

see if te noise is there when stationary. if so hold the revs around 1500 to 2000 rpm if it rattles around that, then it is the heat sheild on the down pipe section thats broken free.

sounds like you have a turbo with a bad bearing.
pull of the turbo and rotate it by hand to see if it is hanging up.
Most turbos start to make a different noise when they start to go.
Sometimes dirt can bypass the air filter if is not changed on a regular basis and can cause the turbo to fail or become dirty enough to cause it to spin slower, (the dirt prevents the exhaust to spin the turbo at the right speed and the air passing the fins can whistle), it will cause bearing failure after a while. Also if you have too much soot in your exhaust from too much oil, the exhaust side of the turbo can become too dirty to operate properly