So, got a 2014 FL 2.0TDI 170hp 4x4 estate which in my opinion seems to be humming/droning around 2200-2500rpm when going around 100-120km/h on the freeway in sixth gear. Does your ones do that too? It's very noticable when sitting in the rear seat making conversations extra hard.

 

Could it be the mufflers that are crap or is it just like this on all these cars?

Rear wheel bearings are common 

Replaced both rear of mine At 120,000

Could also be coming from rear tyres common known as sawtoothing check on this forum for more info

If you rub your hand over the tread blocks you can feel and see the "sawtooth" on them 
The estates are bad for it with the camber going of on full weight..

I replaced the inner adjustable lower camber bush with new bolts and Powerflex bushes and outer top bushes as well .
Then full 4 wheel geometry reset  with Hunter wheel alignment system with someone that knows the correct way to set it up😎

Up at 137,000 and tyres are wearing perfectly and no noise drone

My 170 4x4 does exactly the same, would like to get to the bottom of it. Tyres seem fine, rear spring carriers have been replaced along with camber bolts and 4 way alignment done - no change

@psycho6149

Try and swap the front tyres to rear and see if it makes any difference.

Also tyre choice can make a different noise level

My Uniroyal Rainsport 3 were very susceptible to rear saw toothing . The Michelin Crossclimates I have now are standing up very well front and rear😎 .....