They name thee before me, / a knell in mine ear; / A shudder comes o'er me - /
A 'knell' is a type of bell - usually used in a funeral. This could suggest the idea of when people talk about this woman, Byron is reminded of a lost, dead love.
The knell in the ear means that you are unable to escape from the noise - he is unable to escape from her memory.
I overhear people talking about you and to me it is like:
Byron was known for having many promiscuous relationships, both heterosexual and homosexual. 'Why wert thou so dear?' could link to this idea of him having a number of relationships and asking, why, why was she different to the others?