Hi all .I had a look through posts but could not find any solutions 

My problem is when looking for parts online "particularly drive shafts as mine are noisy"

I keep finding the right item but then the specification is sometimes different 🤔for e.g. the length on one item is is 499 mm and another one is 460 mm 🤷‍♂️ how do I get the right spec for my model.?!?

Let try to establish what a5 you have, auto ,manual ,quattro ,3 or 5 door? Year ?

Ah sorry I should know better .Yes of courses: 

A5 cabriolet . 2010 .2.0 tdi. Manual 6 speed.2 door.

 Started with a very slight knock almost a hum.  Most prominent around 30-40 mph . When I rock the wheel back and fourth something sounds looses with a light knock .but on both sides wheels and tyres are all good 

It does not sound like driveshaft . Best way to test that is on a empty carpark, turn wheels all the way one side and accelerate rapidly then do the other side, if there is a problem you will hear dinstinctive clicking noise . 

It could a wheel bearing that causes the nosie or even brakepads not retracting ... knock on the wheel could be a ball joint 

Best will be to take it to a garage for checking.