Sawtooth wave. It resembles the physical behaviour of bowed string instruments. Friction on the bow pulls the string back, till the tension overcomes the static friction, and the string snaps back; the string eventually "sticks" again to the bow and the process repeats.
Kind of like a sawtooth wave, it ramps up until it hits a certain level, then snaps back to the bottom.
Then play around with filters. E.g. try different resonances and cutoffs to see if you can emulate how a cello or other string instrument has a natural resonant frequency of the hollow chamber it's made of.