FL2 can, at rare times, experience a flutter of the Turbo Vanes.

Mine does it on a couple of inclines when cruising at about 30mph, changing RPM (gear) / Accelerator Position / Speed stops it happening. Very odd but I take it as kind of FL2 feature.

https://www.freel2.com/forum/topic38601.html?highlight=turbo

Another source of RPM related noise is the Alternator Pulley One Way Clutch.

https://www.freel2.com/forum/search.php?se...rch_time=0 FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 77k+ miles) (MY2015)
Metro in its 11th Year of (Extended) LR Warranty / Full LR Service History
(Expensive, but Trouble/Worry free - hopefully?)