Code deleting from Header.php

Hello

I am facing somewhat unknown issue. Not able to find any issue on Web.
Here what I am facing:
On my Godaddy Account I have plenty of domains and sub domains.
On every 2-3 days half of the code deleting from activated theme’s header.php file. And because of that error showing on website. Once I replace the code OR header.php file afterwards website working completely fine.

This thing doing on all my WordPress setups randomly. I have around 5 main domains and around 15 sub-domains.

Not getting any idea what type of execution happening into my hosting OR at where any malicious code OR any Hacking going on.

P.S. : Also tried some well known Security plugins.

Expecting any good solution from WordPress Community.

Thanks in Advance.

Harsh