This is a term used to when going to bed with someone. To make hay means to copulate in bed with a significant other, often not only for sex but for sleep too.
While playing Call of Duty Modern Warfare 2 and you are in any flying vehicle firing upon any enemy force, building, vehicle etc, you are considered to be "making it rain"
"Yo Steady Eddie I am bunkered up in the farm house, make it rain son!!!"
"Let's play call of duty, I want to make it rain!"