It was a pre-dawn, no-knock raid.  In order to get a warrant for such a raid the FBI would have had to convince a magistrate that there was good reason to fear that documents would be destroyed if notice was given to Manafort.
Word is the White House staff was "rattled" by the news.

even more serious, is that Mueller has released his personal financial information and case history to show he has no conflicts of interest.

Could this mean Mueller is getting ready to fire the very heavy artillery?

Could Donnie Jr.'s home/office be next (just kidding, clearly Mueller would not do this to the president's son).