they seems a different jack,a RCA have ground,input and the 3,5mm have TRS/TRSS.
it wont do nothing,except hissing cuz of grounding signal,anyway as long they won't fit in and get stuck inside of 3,5mm ports,it is okay
always set the speaker volume at 30% at least when connecting to source,
hissing wont do anything with your speaker,cuz it due to analog signaling.
hissing sometime occur when your computer microphone section "Listen to This Device" was turned on and theres analog signaling problem that indicate signaling problem or "Microphone Boost was turned on". have you trying to disable stereo mix,or line in/aux,line in or aux in or any input signaling in your computer?
and dont forget to disable "Loudness Equalization",sometime it do boost hissing while theres ambient noises from your computer