A knocking noise could be the fan just a bit out of alignment. As it starts it can rub against the housing, and bang a bit, but with speed the centrifugal force straightens it up and it runs fine. This is an indication of a loose or bent fan blade, worn motor bearing, bent housing, or debris. It is not serious as long as the fan runs silently when up to speed. But adjustments or replacement will be needed.
To clean the coils, any good plastic bristle brush will do, as long as it reaches what you need to clean. Compressed air often is more efficient than vacuuming. Compressed air, coupled with a cowl and vacuum is less of a mess.