Noticed as I drive about 30, 40 MPH thereabouts that there is a noise in the rear of the car like I've left a window open (but they are all closed). Checked tailgate seal but no sign of damage. Read in another forum tailgates on earlier models were sometimes fitted misaligned but mine is square on. Could it be that I need more sound deadening? Not sure whatelse to check next?

 

Just based on my experience... I had a similar wind noise in my Sportbrake. I thought my window was all the way up, but in fact it was not. I'm not sure why it does not go all the way up sometimes, thus create a seal. My solution seems to be rolling all the way down and then back up -- repeatedly pulling up on the switch when it gets to the top. That seems to convince it to slide up that additional fraction of an inch.

Hopefully that works!

 

this happens on a 2018 XF too fyi

 

Why don't you check the noise with the A/C recirculated air on and off?