Mechanistic sound coming from beneath bathroom floor

[sou_oslo]

We have been hearing a mechanistic buzzing high frequency sound beneath the stairs outside our apartment for the last 5 months since we moved her, but since the last week we are hearing a similar sound beneath our bathroom floor and they seem to be occurring consequently, like a paired sound one after the other. Difficult to understand what it is coming from but very annoying and we are starting to suspect something serious