996 Tiptronic Noisy Gearbox/Diff - Advice needed

I upgraded my 964 C4 man to a 996 Tiptronic with about 90k on the clock from a well-known dealer in the Midlands recently. Unfortunately I didn't get to drive it as it had just come in and was on a ramp but drove an equivalent car and was happy enough with that, especially as this dealer had a fair reputation and the car came with a warranty. Very kindly, they delivered the car to me some time afterwards. After driving the car for a very short while I notice a few issues and contacted them. We agreed for me to drive there from London and drop it off & they would deliver it back to me after the problems had been rectified (I had to pay the petrol back!). Anyway, all the issues were dealt with as agreed apart from the main one! I noticed a whine which appears to be coming from the rear of the car when under some load. It isn't noticeable when the car is coasting (or under acceleration because of the engine noise), however, if I apply gentle acceleration at low speed or even driving at high speed on good roads the whine it very noticeable and almost intrusive. Before it was returned to them Porsche East London agreed the gearbox was noisy and had been repaired at some time in the past. The supplying dealer said they took it back to the company that repaired it 6 months ago as was told it was ok and was acceptable and within tolerances. Obviously something isn't right and getting towards the end of its life and was wondering if anyone had similar problems with the Tiptronic Box/Diff and what the problem/solution. Also, any ideas on the comeback with the dealer. If I had driven it and noticed it then I would not have bought the car. Apart from that (and a 5 year old Sat Nav disk), the car is spot on - such a contrast to the 964 and the SC I used to have.