Glad it’s sorted, but TBH your initial assessment didn’t really indicate a faulty hub , after all you even state in the OP that the customer service rep , ‘like yourself’ , couldn’t decide between faulty hub or master socket and I’m surprised that a replacement hub wasn’t sent out when you first contacted BT as the initial ‘fix’ , ( it’s the thing the CSR would normally do in these circumstances) unless by you bringing the master socket into the debate they decided that it would be better to send OR first .
The OR engineer assessment that it could be fans in a data cabinet was clearly ‘odd’ but it’s unclear to me if OR are responsible for ISP’s routers, and presumably OR could have simply synced your line with their own kit , which would have had no ‘hum’ thus proving no issues with the OR equipment and indicating that yours was faulty.
My own initial diagnosis and response was faulty router , that’s why I asked if you had received one or not, the power supply questions were because it wasn’t clear if the hub was OK in the master socket test port , or not.
