Horn Improperly Working and RPM moving on its own

When I purchased this car the horn honked for a couple days then eventually stopped. However one day when I got out and locked it, it honked. That was in March and it has been working since. However it sounds kinda weird and I feel it should be louder, and other impala's of the same models doesnt sound like this. What could the problem be, and where are the horns located. As far as the RPM its veryyyyyy slightly moving on its own. But only when at a complete stop or in park. However when driving it moves normally, and I know it shouldnt be moving without my foot on the gas, and the only time it did this before is when the vacuum plug came a loose, and it stopped after the fix. So either thats messed up again or something else is wrong.