Damage effect from a trap.

Just wondering, what would you recommend be a good damage notifier when you hit a trap? Can I like flash the triggering unit like red for like a split second? If not I guess I'll have to do a sound effect or something...

Cause so far when you hit a trap in my map it just drops your life but you don't really notice until you check your health, though its easy to see when you've hit a trap, just would be nice to have some kind of effect.

Well, depends on what kind of trap you want to have.
I prefer the Impale, Frost Nova, Thunder Clap, Spike Barrier, Liquid Fire, Monsoon and Fire effect, but you should also use a sound which fits to the effect.

Special effects on the unit, fade screens, flaoting tekst, sounds.

those are a few examples of what you could use to indicate something happened because of the fact that you hit a trap.

What type of traps? Just a trap? Frost trap? Fire Trap?

Is that the only/one of few units the player controls? In that case I would go for some scream or "ouch" sound.