Just say SQUEEEEEEE! in a really high-pitched voice. So let the people around you look at you funny. It's Magnus Robot Fighter, darnit!

I always assumed it was the sound of their radio-style speakers being cut-off in mid-sentence (along with their heads, as Magnus was wont to do - made sense anyway, the Pol-Robs' necks were among the thinnest and most vital areas of their bodies). Think of a high-frequency feedback sound on a microphone, or metal scraping against metal.

Had Magnus been made by DC or Marvel, no doubt many more people would have heard of it by now and it probably would have been a film or TV series. Then again, it probably never would have been the Magnus we all know and admire.