Im not sure if the Verizon DVR enables sleep mode for inactive HDs but my PC's esata enclosure will sleep my drive every 30 mins when not in use and it then will spinup (making the noise that you notice) when I access it.
I don't know if it has a sleep mode or not. It is a Western Digital 1TB unit and it was essentially plug and play with the FiOS DVR. No configuration needed. It never did make this noise until a month or so ago.