Same on my Cupra. It doesn't do it when the road is very flat, more so in corners and over bumps. I can replicate the noise when the car is stationary by flopping into the backrest, making sure I press in the middle of my back. I've never bothered the dealers as I'm sure they'd say "Don't flop into the back of the seat". As already mentioned, I usually have the radio on anyway.
Would be curious if anyone knows a fix though.