One of my cats, Joe, will start pacing and howling for what appears to be attention. Over the years I have discovered that he does this because he is suffering from "cabin fever". If he gets pent-up inside for any length of time he'll start. He does this when it's been raining for a few days or if there is a larger cat outside stopping him for his favourite task - exploring. On the few times I have moved with him I make sure he is kept inside for a couple of weeks so he is familiar with the concept that this is now his new home. This always results in stacks of howling...

I have also noticed the howling will stop while he's getting affection but start again soon afterwards.

Has your Russian Blue gone from a sitution where he had a lot of outdoor access (or just a larger area) to one where he has less?