It's a reference to either:
1. A rocket launcher
2. A helicopter
But also can be used for fun.
Helicopters make a soi soi soi kinda noise so saying pharses like the one in the example explains it, the rofl is added on just to make it seem kiddish and funny.
Eg. My Roflcopter goes soi soi soi soi soi (as you spin around with a rocket launcher or just because helicopters make a soi noise)