Mystery whine when steering left

Hello all

I hope someone can shed some light on this issue as it has me, and a number of other mechanically minded types, stumped.

My 2.0D X-Type (64,000 miles) has, within the last fortnight, developed an annoying whine when steering to the left. This starts to happen once the car has been driven for about 8 miles, so I assume that something has warmed up by this time.

The whine is relatively low pitched and happens at all speeds when steering left (either a few degrees or a full lock left circle). It stops immediately when I straighten up. It does not happen when driving straight or steering right.

All of the following have been checked and found to be fine:

Power steering fluid level

Steering wheel wobble

Brake pads

Brake discs

Suspension

Tracking

Tyre pressure

Tyre tread depth

I hesitate to delve deeper without having a better clue regarding the potential problem. Any clues or shared experiences would be gratefully received.

Thanks

Geoff