All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no treatment for such whisperings except ignoring them and giving them absolutely no attention at all. No matter how frequent or overwhelming they are, the person should totally disregard them. There is no doubt that finding whisperings in Aqeedah (beliefs) to be hateful and disgusting is proof of outright faith, as the Prophet, sallallaahu ‘alayhi wa sallam, said. However, what is more perfect and higher in degree is that the person's heart is assured with faith and that he does not experience such whispers. Therefore, this person should not get carried away with these whisperings or give them any concern whatsoever.
As for whether they are the result of Sihr, the evil eye, or otherwise, that is all possible. Whatever the cause for them is, it is not important to know it. Rather, what is important is to know the means to get rid of them. So do not preoccupy yourself with their cause, and focus your attention on finding the way to overcome them, and this can be done by resisting them and ignoring them until Allah blesses you with perfect recovery from them by His mercy and grace. You should also strive in supplicating Allah, genuinely placing your trust in Him, resorting to Him for help, and reciting Ruqyah (healing through Quranic recitation and supplication). Indeed, Ruqyah is an effective remedy for this disease, whether it is caused by Sihr or otherwise.
Allah knows best.