I've been hearing 2 to 3 metallic knocks during hard acceleration since ~15k miles on my 2015.5 XC60 (Drive-E T5). After reading another thread in this forum (link below), I believe those were engine knocks. It makes sense because after switching to premium gas from regular, I hear the knocks less often and they were much less noticeable.
Here is a thread on the S60/V60 forum where owners are complaining about the same thing, and as of today it has 300+ replies. If anyone else is experiencing the same thing, please chime in! I'd love to see this fixed.
EDIT: People in the thread have posted some videos. It should sound like this: