"Nagging fear" refers to a persistent and bothersome feeling of worry or anxiety that constantly reminds you of something you fear or are afraid of. Full definition
In fact, it may not take much to help him overcome nagging fears. (babycenter.com)
That's the nagging fear of advertisers who «play» the trade publications. (inc.com)
Certainly I harbour this nagging fear that one group or other of fundamentalists is correct and being trapped between the Scylla of eternal damnation and the Charybdis of a monstrous God of hell - fire, brimstone and Old Testament genocide. (nakedpastor.com)