Externally: It takes something loudly rusty, nail being scratched on a chalkboard kind of grind to actually bypass my sensory thresholds. I'd even sensory seek at pouring alcohol on superficial wounds.
Otherwise, it's a situational kind of grind: being suddenly bothered by any intense stimuli while focusing. Or anything similar to being splashed with cold water in your sleep.
Internally: All I have to do, is chew something that is generally too hard, too rough or slippery to chew: which is many things. These includes onion pieces of any sizes, carrots/beans/seeds that aren't soft enough, etc.
Anything that feels dry and sore.
The emotion of envy, sensing belligerence and haughtiness from others.
Some sensory issues I got doesn't 'grind', more like it accumulates, barely bothering until it's uncomfortable or just takes out from me for some reason.