Jemax,
Had that one long time ago in an old R22 Beta, it sure gets your attention the first time it happens eh?
Especially if you are solo, or low on experience at the time.... I think it is some sort of interference from the radio, somehow activates the low RPM horn and light, cant remenber how we solved it at the time other then to pull the CB or dont use those frequencies, not very helpful I know, but if you get your avionics man to have a look, im sure he will be able to solve it. I am guessing it was an R22? if so how old is it?
Regards,
BC