You know when you leave a glass bottle outside and you can hear the wind pass the top or like wind blowing through a tunnel.

 

The sound gets more noticeable on rougher road services too and is coming from the rear of the car.

 

It might be a huge coincidence but I had my exhaust spot welded to the very rear clamp as the exhaust was broken free from it before but I believe it maybe bearing or just tyre noise that I hadn't noticed before.

There's a reason your exhaust usually hangs from rubber bungs. I would guess that exhaust vibrations are travelling through the spot weld and into the chassis of the car.

Doesn't really tally with these change in road surface though, that would point to a suspension or wheel issue. Is it worse when taking a bend? Does it vary with speed or with revs?

Could be a wheel bearing if so...

There is a rubber dampening bung, when I take it in the garage I will get them to check the bearings and springs for any movement or vibration at the rear too. I notice it even if I just accelerate forward 10 to 20 feet.

Is it one side or both?  Even in the relatively quiet Mk3 a poor road surface causes a lot of tyre noise without there actually being any fault, there are several fairly small (few hundred yards) patches of dual carrieageway replaced round here that almost silence the car for a few seconds lol.

It sounds like its coning from the rear left exhaust side to be honest but like you said when on the soft road bits it goes away.

To be fair, there's a stretch of the m180 between the a15 and the Humber bridge that's a beige colour and sounds like you're in a wind tunnel...

I had something similar on my old car. 

Only happened at 60mph and was caused by uneven tyre wear - “blocking” on the outside edge of the tyre. 

Fair enough it was a 4x4 with off road tyres but it can occur with 2wd cars too, may be caused by suspension geometry being out. May be worth looking at the tyres?