It may be coming from theme or plugin you are using. Sometimes developers forget to remove any echo statement (or any other debugging approach) which they use while developing.
You can first disable all plugins. If this still comes up, then it is somewhere there in the active theme. If this goes away after you disable all plugins, then try enabling plugin one by one. This way you can know the plugin responsible for this.
If you have technical knowledge in wordpress/php, then you can fix this yourself. Else the better option will be contacting the plugin author and ask them for a fix. This will help other users who are also using the same plugin.