PSYS_SRC_PATTERN_DROP? What's up with this?

Why doesn't PSYS_SRC_PATTERN_DROP work?

I mean like for a rain type effect?

I need some sleep.

Well, I can only assume your water drop is just hanging out and not "dropping", in which case you'll want to add :

PSYS_SRC_ACCEL = <0,0,-2>

Or something similar, that -2 means "Push this particle along the Z axis, downward", so it should be moving for you if it isn't already.

And just incase you haven't been introduced to it you can find more info on things like rate and age that will help you fine tune it towards the effect you'd like at the wiki:
Wiki Particle Page

Or if you're still having problems after that, post your script here and I'm sure someone will give you a hand.