A little more info: strings, string height, slide type & thickness? But it sounds like technique to me? Does your free fingers touch the strings behind the slide ( this quiets slide noise?) Maybe a few pics of your string action or a sound/video sample? The strings don’t matter, the manner in which the slide contacts the strings is what does?
A little reading about strings:
Sometimes you want the extra string noise, sometimes not. If not, then, as Brian Q mentioned, employ string damping.
This explains it well: