As others have said,try switching off the 'Anytime TV' option.
Switch off the 'auto stand by' function too.
This should make your fan quiet again.
If not,try doing this AND a forced software upgrade.
To do this,put the box onto standby and switch it off at the wall.
Press and hold the 'back up' button ON THE BOX.
Whilst pressing the 'back up' button ON THE BOX,switch it back on at the wall.
Wait a wee while and all the lights on the box will light up,perhaps accompanied by an on-screen message saying "software upgrade".
After about 10 minutes,the box will go onto stand-by itself and you've upgraded the software.