Dealt with a similar issue on my old 06 Z71 before it was totaled. Gonna say it’s your water pump weep hole, all of the coolant would seep down and drip behind the pan. Literally just had this job done 3 months ago (2 weeks before my wreck...) change out the water pump ($110 on Amazon & $200 in labor if you don’t do it yourself) and you should be good.
Hope this helps man!