Hi Collette,
It's definitely coming from the box. I think it's the fan or hard drive, as it starts when I turn on the box and also sometimes when it is on standby. My old V+ box did a similar thing but it was much quieter, almost inaudible. This one is louder and much more intrusive, especially if watching a quiet programme.
Having looked at some of the previous posts on this forum, it seems that there is a range in quality of available V+ boxes, which are presumably all reconditioned/second-hand ones as they are no longer being made. I will either get it replaced or move on to a TIVO box, which don't appear to have this problem.