hissing sound on speakers

what causes the hissing sound on my speakers every time i rev up my engine?

Be a little more specific.  It makes it more difficult to troubleshoot if we don't know any specifics.  Tell us the year/make/model, along with the gear you have installed as well (brands, descriptions, and model numbers if known).  Is this a stock setup?  Is the problem isolated to one source (only on radio, or only on cd).  Is it only on that cd/track?  Basically, when does the problem occur?  Be as specific as possible- is it only at loud volumes?

Also include pertinent information like: when the problem started, if it gradually got worse over time or if it happened suddenly, if any work was recently done to the car, etc.  Did you have the equipment professionally installed?

We'll be glad to help.  Thanks.