The bkgrnd ck may take many days or month

The bkgrnd ck may take many days or months, as long as you knew you are clean, nothing to worry about.
If they said you can start work now, go start it.
If they say you have to wait, then keep it as a secret, and wait.
Too bad your current manager already knew you will leave. In this case, don't be horry, wait, wait, wait... untill the new company say you can start.

Thanks!