Glaive is helpful for this, too. You can't aim it up or down, and the lack of a crosshair makes throwing it a bit difficult, but because it's silent you can literally kill targets under the eyes of other targets without being detected, provided you're hidden properly.
Ash's Shurikens are silent and can be used while invisible. I made a build combining Energy Siphon, Smoke Bombs, Shurikens, Streamline, Fleeting Expertise, Continuity, and Constitution. The invisibility lasted long enough that I could waltz into one of the areas and just Shuriken everything to death, and they were so cheap that Energy Siphon provided more than enough energy.
Ash's Shuriken is silent, you just have to make sure that no other Grineer see the kill. Really though, you shouldn't even need it. Just have patience and the test is pretty easy. Don't rush it, just wait for the right time to strike.
Teslas are silent and therefore viable. However, if a Grineer sees another die, you lose, so be careful.
As a side note, Vauban's a pretty interesting option for stealth. His abilities are all silent, he can lock enemies in an unalerted state, and Bounce can help you get places where you won't be seen. He's currently my favorite non-traditional stealth frame.