Skoda Octavia 2008 1.8TDI noise on accelerating

I've got a 08 Skoda Octavia Elegance Estate 1.8TDI Automatic that's done ~65,000 miles.  Recently I've noticed when accelerating I can hear a whistling/whirring noise from under the bonnet that gets louder when accelerating and then when at constant speed it lowers in volume. I did try listening with the bonnet open while increasing the revs and it didn't appear to be coming from the alternator and sounded more like it was from the back of the engine compartment.

I've not noticed any smoke from the exhaust or the car using oil and have no warning lights on.   It makes the same noise when standing still in neutral and increasing the revs.  Not sure if it's relevant but some time ago the air conditioning made a slight grinding noise which stopped and the garage told me the clutch on the aircon pump had failed and just meant the aircon wouldn't work but wouldn't cause any further harm, As it was pretty expensive to replace I didn't.

I took it into the garage today thinking it was something like the alternator, water pump or turbo and they took it out for a drive and think it is coming from the gearbox.  They asked me if the gearbox had been serviced and gave me a number of a gearbox specialist to take it into.

I'd appreciate any pointers on what this could be or further checks I can do to see what the cause is. I'm hoping its not the gearbox as I've heard that can be pretty expensive to resolve.

Thanks

Martin