It could be your air handler If it is up there. You need to go check around the unit , especially the drain pan . If there is no water in the drain pan then make sure the inside of the unit is draining correctly to the condensation line (pvc pipe)

If all that is fine then make sure the unit is level, sometimes a unit that is not level can have draining issues and will leaking can occur.

If you are absolutely sure it's not the ac unit then go up on the roof and check shingles, they are cheap. If you hire a roofer or hvac guy to check the roof and system you will get charged, a lot .