Focus 1.8 diesel. Has started with an oddish noise that I've never quite heard before. It's a high pitched rumble noise, engine speed dependant with a whistling component as well.
Tricky to nail it down, but slip off the aux drivebelt and it stays so it's not the water, PAS, aircon or alternator bearings.
At idle there is no noise - it only comes in at about 2500-3000rpm plus. It doesn't appear to be related to engine load.
Any ideas? All I can think of is a problem on the timing belt, turbocharger or something unpleasant like incipient little end failure or piston slap. It doesn't sound like big or mains going (heard those before).
TIA.