This problem just started happening since I got to my new apartment. I've had this computer (2014 maxed out MacBook Pro) for about a year, and it never happened before moving here.
Basically, when I am using the computer to browse the Internet or work on docs etc it is quiet and responds normally. When I stop using it for a while, but leave it on (or in sleep mode) it enters very intense activity. Or at the very least the fan starts running like crazy.
The activity/fan only stops if I power down the machine completely, or if I start using the machine again, in which case it gradually slows down (the fan noise) and then stops completely after about 20 secs of me being on the computer again. It is counterintuitive that the computer fan / activity picks up when I am NOT using the machine, rather than vice versa.
I have been able to find very little that is helpful based on Google searches, although I did find at least one post suggesting that this could be due to botnet infection, which was my initial thought and concern (prior to seeing that post).
Probably not much that can be done diagnostically via a message board analysis, but I thought I would post since there doesn't seem to be a lot of easily available info on this phenomenon. I suppose a less nefarious explanation would be that there are major updates that need to run in the background, but that explanation doesn't seem consistent with my experience using Macs previously.