Probably a relay. Are you comfortable upside down? Get under there with a flashlight and a screwdriver, and when clicking hold your screwdriver on each relay in turn. You should hear or feel a difference when on the right one, or of course if it does it continuously you can just pull one relay at a time, though some relays may receive power through another so if the sound stops it does not necessarily indicate that relay alone.
You can also try swapping relays around when the same type.
Does it eventually stop after some idle time out period from a battery saver circuit? As above that would not necessarily indicate the battery server relay but could be, or would possibly indicate something operating through it that loses power once the battery saver circuit times out. Remember that many actions, opening a door or hood (if there's a hood switch), disconnecting and reconnecting the battery, etc may reset the time out period for the battery saver so start a new waiting period till it's unpowered.