harmonic humming at motorway speeds

My 2014 Superb 4x4 3.6 estate makes an odd 'harmonic' type 'veroom' at around 65 - 75 mph (indicated). It's like that sound you get when slowly overtaking a truck and engine revs are almost the same as the truck's. In a previous life I used to often drive boats with two big outboard engines. We'd use this noise to sync the engines, which was more accurate than using the tachometers or just putting the throttle levers in the same place. When the revs were the same on both engines, the noise ceased.

I'm getting the same on the car, it's not at all loud or intrusive.

The 'veroom veroom' is very low frequency, with a 'veroom' per half second or so. I can't pinpoint it being front or rear or both.

It is definitely road speed and not engine speed, as I have held it in 5th and even 4th, and the sound's exactly the same. I even got it up to the higher speed where it happens and put the car in neutral and let it slow down to the lower speed, and it was the same.

The car's just passed an MOT with no advisories, but I don't know how much they look at the drive train other than for leaks or things falling off etc. I changed the transmission oil and filter about a year ago and also the Haldex oil/filter and the diff oil both ends (the old oils were old but not horrible) - 6,000 miles since then. There is possibly excessive lash at the rear diff as it is a little bit 'clonky' when taking up the slack at slow speed.

Anyone experienced this or have any ideas as to where I should start looking? (Thanks in advance!)