I had the heater core replaced in December. The noise started the day I picked the truck up. After it has been sitting (overnight, 10 hrs of work), I'll start the truck and engage the A/C. Most of the time there is a crunching/grinding noise coming out of the front end for about 1-2 seconds. The noise only happens when the A/C is engaged after the truck has been sitting. The A/C seems to only get cold when the truck is moving. When idling the air doesn't seem cold. Most mechanics say it sounds like a compressor clutch, I find it hard to believe that this is just coincidence that it started immediately after replacing the heater core. Any suggestions? Could the discharging and recharging of freon for the heater core have anything to do with this noise? Any help is appreciated.