First of all:
Guten/Schönen Morgen noch is not a greeting, it's a parting phrase. It's the
noch that makes the difference.
noch just means something like
for the rest of your day from here on. It can be quite an impression of some familiarity or intimacy to express your closeness with someone thorugh even your future thoughts for this person.
Most often it's just an empty phrase of everyday's obligingness... That said it then might be quite obvious that it also can mean the exact opposite when you say
Guten Tag noch in an agressive or ironic voice. Maybe the Berlin bus driver treated you like a piece of clumsy baggage. When you get out at your station you might wish him a
Schönen Tag noch emphasizing
Schönen, meaning you wish him a bunch of football hooligans singing and dancing in his bus until the final station ;)