[b3lha]

There is nothing inside the ECU that can make a noise. But I've had the rapid clicking on one of my cars when I damaged the ECU.

There's a relay, controlled by the ECU, that provides power to the ECU. Basically it allow the ECU to reboot itself if it's not happy. On my ECU, the RAM went faulty and it did the rapid clicking thing exactly as you described. Rebooting over and over.

I guess it's possible that same thing might happen if the ECU has insufficient voltage.

I think your best bet it to find out which relay is clicking and look it up on the wiring diagram.