Omen has recently added it when you are close to getting aggro I believe.
Correct, the related default sound effect kinda shocked me too the first time I noticed the option added, I promptly disabled the warning afterwards since I either know I have aggro or find out by having a red dot appear next to my 'box' in Grid.
What is the mod under the target's unit frame? It's driving me nuts b/c I can't get rid of those timers. I already have DoT Timers and don't want those but I can't seem to figure out which mod is doing it. Here's a list of my mods: