Whining Noise - Help Please

Dear all,
Over the last few weeks my car (840 ci) has developed a whining noise coming from the front of the car. It is only noticeable on deceleration (not braking) and slow acceleration. It is noticeable when driving in straight lines.
It is an automatic, gear changes are fine with no clunking or hesitation and no messages on the OBD.
I had the transmission and differential oil changed about 20 months ago at Phoenix. Prior to that there was a small leak, I think it is still there, but not much more than previous. I had a new stereo fitted 10 months ago and the engine and check engine lights have not come on since when starting the car (I was told it was most likley the bulbs but have not had time to check).
I would like to take it on a long journey but a bit reluctant to as I do not want to take any chances.
Any thoughts would be much appreciated.