This article explains some of the conditions required for the burble to occur:
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On the Cupra, this is more prominent in Sport and Cupra modes only.
In terms of the burble, there could be many reasons why it’s not so prominent, such as exhaust/cat temperature or the type of fuel - some people state that different fuel brands have a different impact or the differences between winter/summer fuel grades.
In addition to the driving modes above, quite a few people have asked if the mode changes after a set period of time eg 30 mins as the burble lessens after a longer drive, in a similar fashion that the exhaust note changes after a cold start.