[QLDer]

It sounds like a whine (or even a turbo whistle) that follows the engine revs from above 1500rpm. Can be heard inside the vehicle when driving with the radio off (radio at conversational level about matches the whine volume). At some point in the 2000s the engine noise drowns out the whine. The whine happens every time the engine revs above 1500rpm (not a sometimes noise in my experience).

I lifted the bonnet and held the engine revving above 1500 - sounded like the noise was coming from the RHS of vehicle towards the firewall (near the turbo).