Is it very high pitched or sound like a 4X4 going past ?
High pitched could indicate one of your turbo pipes is leaking if it is a 4X4 sound it is normally gearbox related.
If it is on a left or right turn then it could be a worn CV joint or if a constant metallic squeal/rumble worse on bends could indicate a worn wheel bearing.
If it's non of the above I will keep guessing. I:
Unless your engine is noisy in low gears and when quieter at high speed you are listening to your turbo.