Threshold is the amount of time something has to be in the scene before an alert is raised - so set at 0 - it's going to go off very quickly when it detects a lot of activity, so I might try moving your threshold over to 2 or 3 seconds. In reality if a human comes to loiter, they'll be on screen for at least 2 seconds - so that's probably where I'd be starting.
Rain is lots and lots of moving objects filling your screen in a short period of time, so it will be difficult to eliminate it completely. Check your camera shutter speed - if it's too slow, the rain drops become long lines, if you can speed it up and still get the same illumination at night that might reduce the issue. Most cameras I've connected recently have a default shutter speed of 1/25th of second. For all of my installations I'm able to get away with 1/100th of a second and maintain my exposure - faster shutter speed = shorter rain trails.