I get sticky relays often but not ones that click on and off rapidly, i can only assume you have a micro break in the connection to that relay, else make sure you are not using a pin that is doing something else or is affected by a library or in pwm mode. to make it simple swap that clicking relay digital pin with one that is WAI and see if it stops if it does its the digital pin or code effecting it else if it still does it try a different cable else look on the pcb tracks for a break or lose connection