i got a 07 cobalt ss/sc, i was driving around a couple days ago with a few friends and i wasnt romping the **** outta my car just a regular cruise and at one time i shifted into 4th and it would pop in with such ease but then it wouldnt stay in 4th gear, i could put the car into gear without even holding down the clutch, and usually i can feel when it pops into gear but this time it would go in so easily, its very hard to explain. At the same time though the clutch was not slipping or anything and the car has like 23k miles on it, sometimes i would throw it into 5th and it would do the same thing and then make a noise like it was a bunch of nuts and bolts in a tin can and then like instantly go away after 2 seconds and it drives perfect.

 

Search for "shift cable loose", the end at the shifter likes to pop out.