Caprice

I took my car in for Maintenance. Within an hour of getting it back, I heard a rapid, non-stop da da da da da da under the hood after it had been sitting - the noise stops when the key is put in the ignition, but starts again after the car is parked and cools for a period of time. It comes from both the EFI relay and the A/F Heater Relay. The second one only clicks if the EFI relay is plugged in.
The mechanic is currently trying to find the cause. Has anyone else had this happen? What could cause it? What are these relays linked to, that would need to be looked at? It seems EFI and Air Flow will have some common components. Could anything from standard maintenance have set this off?