Is it ensure, insure, or assure?
Old English cnyll "sound made by a bell when struck or rung slowly," perhaps of imitative origin. The Welsh cnull "death-bell" appears to be a borrowing from English. For vowel evolution, see bury.
Old English cnyllan "to toll a bell; strike, knock," cognate with Middle High German erknellen "to resound," Old Norse knylla "to beat, thrash;" probably imitative. Related: Knelled; knelling.