"Accountsd wants to use the 'iCloud' keychain"
A reboot solved this issue for me.

I had this same pesky popup with no apparent source. There were actually two of them:

acountsd wants to use the "iCloud" keychain.
Address Book Source Sync wants to use the "Local Items" keychain.


Clicking 'cancel' on the first one would cause it to reload instantly, so there was no way to opt out of responding.

I closed all open applications and restarted, apparently resolving the issue. Several things make me nervous about this: (a) The persistent behavior of the modal, (b) the lack of clear source and reference,  and (c) the request for password on keychain which I never use. Is this a new virus?

Here is a screenshot: