How to run a script on login whilst being hidden
Instead of launching Terminal You can make something like:

repeat
    delay (random number from 1 to 5) -- Change the delay in seconds
    do shell script "say Hello"
end repeat


You can save it as an app and then change its Info.plist file to hide it from Dock. Simply add this line inside Hello.app/Contents/Info.plist:

&lt;key&gt;LSUIElement&lt;/key&gt;
&lt;true/&gt;


To launch it on login You can add this app to Login Items.