Front Differential on 2002 C4S

[ifm]

I have just returned from a trip down to Tuscany in my 02 C4S :driving: . All went well except that on way down through France into Italy it developed a speed related whine above approx 35mph(it would get louder as speed increased). It wasn't too loud but loud enough to be annoying so took it in to local Porsche centre in Arezzo who were very good. They diagnosed it to be front differential, ordered part and I took the car back to have the work carried out a few days later all under warranty. A replacement car was supplied by Porsche Assistance although this turned out to be more stressful than the problem itself! What I would be interested to know is this:- has anyone else had any problems like this?, is some degree of transmission whine acceptable at very high speed?, and could there be an underlying problem which may have caused it in the first place? The car has only covered 17000 miles in total and was bought used when a year old from OPC in Guildford with 5500 miles. It has been serviced annually at OPC Guildford and Tonbridge. The only other work I have had done is reeplacing all four tyres with like for like Continental SportContact 2. Any suggestions?