Saying you need to get something off your chest is simply telling someone you need to say something that has been bothering you. If I hear someone saying that, I'm thinking they're opening up about something and want to say it to me for support. Said support usually comes from a discussion afterwards.

If the mechanics of what you're needing is to just say it out loud as to alleviate the frustration within you, then do it with yourself. Otherwise what you're suggesting is more on the level of a counselor or someone who has known you a really long time who fundamentally understands what you're doing.

The average person who does not fit those definitions will not feel invested in you to give of their time to listen to your issues, if you do not value their time to respond in kind.