whine from gearbox?

my 04 octy vrs,108k has a intermitant whining noise from n/s/f area at around 40 mph,car has had in last 4k new front wheel bearings & a clutch & flywheel,gear selection is fine,also the box does whine sometimes when in neutral with foot off clutch(noise goes when pedal pressed)the noise at 40 doesnt change/go if you dip clutch,change direction,change gear etc,like i say it doesnt always do it! theres no noise at 60+,could this be the dif bearings going? is it worth changing the box oil? thanks.