Sorry for replying so late, I must have missed your post
BTW, it would have been easier if schematic were available.
I guess you are misfiring triac. Triac gets turned off by itself when voltage applied get below threshold. So you are right in delaying the gate firing to control the phase angle, but then why are you waiting for "300mSec" once you have fired the gate to turn it off. You must fire on both +ve as well as -ve phase of line. If you continue firing on either only +ve or -ve phase, transformer will ultimately saturate due to flux imbalance. That's what is causing the 'big sound'. It not the magnetising of transformer but rapid & untimely collapse of it.
You should fire on each zero crossing(+ve & -ve) after desired delay & remove gate pulse well before next ZCD.
Better simulate your design & code, your transformer might get damaged.
BTW, I must also add that ISR is usually not a good place to reset the watchdog timer. It may happen that your main-line code fails & now requires a reset, but ISR that continues working unknowingly will keep on resetting the watchdog will just void the very purpose of watchdog. It's better to use main-loop to reset the watchdog
Hope that helps...