"Herein, Bahiya, you should train yourself thus: 'In the seen will be merely what is seen; in the heard will be merely what is heard; in the sensed will be merely what is sensed; in the cognized will be merely what is cognized.' In this way you should train yourself, Bahiya.
There is no knower per se, as can be seen from the above, in an ultimate sense, there is however a process of knowing/or there is knowing.
The 'knower' belongs to atta-ditti (self view) category. It is a false projection based on the five aggregates, which do exist in a truer form (comparatively).
At best we could say there is consciousness and then there is name and form, to denote subject and object.
When we experientially sense a knower it is merely (usually) some arisen name (mental) elements we mistakenly identify as a knower.
When we see something and then when we turn to see 'the knower' there is a error being made. We assume 1) that someone is seeing therefore we try to find something to tag as 'the knower'. 2) we assume that dhammas/elements do not pass away immediately (ie that there is continuity), and is therefore meaningful to look for a knower, after the act of knowing has passed away.
Anything that we identify as 'the knower' if we sit long enough with it, fades away. This would be one way of overcoming the ignorance of the (continuous) 'knower' who is resident within. The other is to keep noting that knowing arises based on (for example) the eye, visual object giving rise to eye consciousness. There is no resident consciousness which 'picks up' these images. Consciousness itself is causally arisen. When the cause doesn't exist consciousness fades away. From this it can be seen that what is at work here is a biological process of perception. Nothing more. If something more seems required, then I would suggest the ignorance of sukkha is at work.