Just wanted to point out: Detonation (Knock) typically sounds like it's coming from the head because it's happening directly underneath it. The block absorbs more sound than the head typically does, so it channels up the explosions. Essentially there's no way to tell exactly what kind of sound you're hearing without a massive amount of time spent with your particular engine as they die. It could be bad bearings, pre-ignition/detonation, an issue with the valves, a loose spark plug, or as was the case in my '93 accord, it could be the sign of a cylinder starting to go bad. In my case the piston rings went bad in number 2 and caused blow-past which overheated the piston rod and burnt the crank bearings, melting the rod and causing it to warp and causing the bearings to seize and shear.