D40 Vibration

i have st d40 manual only 22,000km with vibration/noise thinking of selling it. problem started at 17,000 apprx everyone said its tyres. tyres have been replaced its not tyres its intermittent shaft checked by nissan said ok. sometimes feels like car is been held and tried on cruise control feels the same like something holding it and can feel vibration under feet. steering does not vibrate. sometimes feel like something not turning round. problem cant be found thought maybe cv joints but not sure anymore. nissan will not try and find the problem unless it becomes really big the vibration so if it vibrates a bit you stuck with it. anybody have any idea what it can be? if it was a cv or bearing to do with front shafts why does it do it then can settle down a bit for a while a bearing would play up. what else could it be?