Super Junior's Leeteuk and f(x)'s Krystal have teamed up to deliver a sweet new song for the "Enjoy Today" ballad project! Titled "Grumbling", their soothing voices compliment the sweet melody and the mid-tempo beat is sure to make you sway. This song is must-hear for fans! Check it out below: Here are some pictures from the set of "Enjoy Today" with Krystal and Leeteuk. Thanks to blog.0421 for the tip!