Please help diagnose this mystery sound...

[Bananas]

Hello
I recently became the owner of a 2011 D4 2.7 which is creeping up on 340k. It has been owned since new by my parents who have looked after it very well. It has been their adventure rig for the past 10 years. The body interior and exterior are in immaculate condition considering the age and mileage. Parked next to a new one the only major difference you notice at 10 paces is the shape. I have no idea how they have kept it looking so fresh.

But with close to 340 on the clock it's not without it's noises. I'm working through issues and deciding where the comfortable intersection between mechanical perfection and budget exists. Expectation vs reality.

In the 2 months or whatever I've had it so far I've replaced the heater matrix and solved a clunk up front. The heater matrix was almost 100% blocked with radiator stop leak and the clunk was a sway bar bush. These were the main noticeable faults. Now I fear it's on to the more difficult to diagnose and potentially more costly ones. I have already done a fair few k's on it and I enjoy driving it very much.

There are 5 things on the nice to solve list (so far):
1: Slight shudder when cruising up a hill and the car shifts down - I suspect torque converter;
2: Harsh shifting - not so harsh that would make a non mechanically minded passenger comment or even notice, but not silky smooth either - perhaps a fluid change will heal it - perhaps something more;
3: Backlash in drive train - mainly a bit of a clunk when powering back on after coasting;
4: A noticeable vibration that kicks in at just under 120 km/h. Drop back to 115 and it's gone.
5: Some vocal angry bees or cicadas somewhere.

And it's #5 I'm fixated on understanding now and this is the one I'm hoping for some guidance on where to focus my suspicions.

The symptoms I have noticed are these:

At about 60 km/h up to about 120 when #4 kicks in, a pulsing hum begins. The hum is a bit like a swarm of bees but it pulses like cicadas - perhaps pulsing 3 to 4 times a second continuously.
It is only noticeable on very smooth tarmac.
It's there with the power on or off.
It seems to grow louder as speed increases.
I think the rate of pulsing stays roughly the same at all speeds.
It disappears on a sweeping left bend and returns when approaching straight ahead and it remains on a sweeping right bend.
It becomes less noticeable almost to the point where I'm tempted to say it disappears when powering up a highway hill for example this one:

"The Mt Ousley hill is about 50km south of the greater Sydney metro area and runs down the scarp to Wollongong. For the high volume of transport companies running freight between the two cities, it's the only way down. The average gradient is 9.6 per cent and the maximum, near the bottom is a dangerous 25.7 per cent."

It is louder on the bottom of a dip when you are heaviest in your seat and becomes quieter when bouncing over a rise.
It seems to be more biased on the drivers side - but I have not been in the passenger seat to compare.
It is kind of like tyre noise but less slappy if that makes sense, more reminiscent of a background noise familiar in sci fi films: Waow-waow-waow....

So, what do you think I should suspect first and how would I go about narrowing down the suspects? Is there another way than simply pouring $$ into transmission components 1 by 1 till it's gone? Should I accept it's part and parcel given the mileage?

The easy fix is turning the music up but even then the low frequency waow-waow-waow creeps through when I hit a smooth section.

[V8Ian]

The first thing I'd be looking at would be wheel bearings.

[loanrangie]

For #5 , how snug is the bonnet when its closed ? i have just noticed a chirping like noise emanating from my bonnet that i can hear at idle when standing beside the vehicle but i'm sure the AT's hide it while driving - still need to see what adjustment there for this.
At 340k the shifting issues could be wear in the trans or in the valve body, for me i started with a fluid flush then later a metal pan/filter conversion then 18 months later a valve body rebuild with new solenoids,sonnax zip kit and separator plate which solved all my poor shifting issues.
You have 110k more then more than me so i'd be inclined to say that the torque converter is likely getting tired which would account for the shudder and possibly the vibration.

[Bananas]

Awesome V8Ian and loanrangie

Bonnet seems snug.
Agree wheel bearings are a good one to cross off the list as the car is on the original ones.
Likewise with the torque converter.

I'd love it to be the wheel bearings given these are easy to replace, but I've got a feeling in my bones it's more likely diff, gearbox etc.

At work I occasionally use a big toolmaking lathe which has an unbelievable amount of gears in it's transmission and it has a similar pulsing sound.

[PerthDisco]

What’s the interior headlining like or has it been done? Mines been largely garaged but is really in need of doing now.

What servicing has previously been done on the gearbox and diffs etc?

[Bananas]

Headlining is like the day it left the factory, with a few dirty scuffs here and there. But no hint of sagging yet.

I'm not sure exactly what's been done and when. I'll check with Dad. He tells me that the front lower control arms have been replaced a few times and I think it might have had a reconditioned trade in gear box at some stage through Graeme Coopers in St Peters NSW. I don't know about the torque converter.

He's been pretty good with fluids, both correct types and on schedule but I think diffs, transfer case, wheel bearings, steering and suspension components are original and have not had major work. So there could be a bunch of contributory causes.