Was driving a 10-year-old Urvan in the expressway and there's the usual "tick" sound at regular interval when the aircon is on. However, halfway to my trip, the "tick" sound of the aircon came in rapid succession so I slowed down (don't know if slowing helps but the rapid "ticking" stopped). The rapid "ticking" happened one more time during the trip but I never turned off the aircon and by the time I got home, it was again "ticking" in regular interval.

What is that rapid "ticking," what could've caused it, and how do I fix it?

Thanks very much for your help.