In summer, due to excess usage of your air conditioning system, you can see some water droplets around the ac ducts. This is called condensation on air ducts. Also, some other reasons may be responsible for this like lack of insulation, mechanical problems, and much more. To know all about these causes and how to stop condensation on air ducts, listen to the full episode or go to the link.
https://oombler.com/how-to-stop-condensation-on-air-ducts/