At slightly less than 40k miles, my 2015.5 XC60T6 (e-Drive engine) was making rattling sounds and a "howl" sound. It turns out that there were two major problems: (1) The bearing from a timing belt pulley had leaked its grease and seized thus requiring the timing belt operate with a fixed pulley; (2) Ball bearings in the balance shaft module had fallen out of their race and were rolling around in the tray at the bottom of the engine. Either of these issues could have eventually caused catastrophic engine failure via destruction of the timing belt (which would destroy the head) or seizing of the balance shafts, which would have broken parts internal to the engine. Repair costs were about $4000.

Be advised that it is important to address new engine noises and vibrations. Now that it is repaired, the engine seems to be running normally and I can hear the supercharger whine again - the other noises had been drowning it out.