Virus on my MCP?

For some time now, while using my Mac, I have been receiving annoying pop-up

windows with text on my desktop that only appear for a few moments. It doesn't happen often

but when it happens it is a real torture. They appear briefly with a whistle.

Apparently, there is no relation with any of the installed applications.

Only, by running ClamXav, I find the file "Unix.Ransomware.Evilquest-9757372-0" 

(file associated with Facetime.app), as an infected file.

Unfortunately, I could not find this infected file in the quarantine folder, even using 

the view commands hidden files [CTRL + SHIFT +.] from the system root, or the other command from a Terminal window: "defaults write com.apple.finder AppleShowAllFiles TRUE; killall Finder"

So I couldn't remove it.

Can anyone help me?

P.S.:

These are the Machine/model info on my Mac: MacBookPro8,2 with macOS High Sierra version 10.13.6

Thank you very much for any suggestion/help you can provide to me.

Massimo

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