Make sure you have removed the heat sink of subwoofer drivers and the 7 screws.

Unsnap at least one connector that links the main board to the woofers.

Spot the capacitor that you will need to change. Spot the shape of the top of the capacitor : if in curved shape it means the capacitor must be changed asap. Sometimes there is no visible damage of the capacitor but the replacement of the capacitor will still fix the sizzling sound.