Not convinced you should hear a 'clanking'. I have owned my 964 RS for 10 years and from what you describe, it sounds like the diff may need some attention. Best to get it checked by a specialist because a diff rebuild is in the region of £1,800 to £2,500 depending on parts needed etc. I know of many cars with that mileage and less that have needed a rebuild. Doesn't mean it's a bad car though! One good way to test is to reverse the car on full lock. If the diff knocks, it's knackered.