2008 Chevrolet Impala A/C unit?

If I'm driving down the road with my air on an say I turn the A/C off (but leave the fan on) it starts knockin over by like the glove box area. As soon as I turn the air back on it works an the knockin/tickin stops. If its on A/C an I switch it from recerclote air to fresh it again starts knock in/tic kin over by the glove box. An final if I have it on the whole time get where I'm goin an shut my car off it'll do the same, all times lasting bout 5 min then its done! What the heck is wrong an whats it cost to fix?