Well i had something similar though it wasnt the ECU but the BEM.

My whole interior is out so i went to plug just the loom back in so i could start my car. (but no dash, just the wiring)

Plugged everything in (i thought i did) and without even turning the key i heard the BEM ticking.
I have started my car this way several times before and had never heard this ticking.

A minute later i realised i didnt connect the ground wire with the green screw that belongs in the middle, behind the dash near the windscreen.

As soon as i screwed that back into the metal the ticking stopped.
Just to make sure, i pulled it back off and the ticking started again.

Hopefully its that simple for any of you. Maybe a ground wire or any other wire has come loose?
Or has anyone played with any of the wiring around the ECU?

But maybe my case was different considering it was the BEM and not the ECU.