True, but "unreasonable" is a nebulous term that the SCOTUS has basically defined over time. There's no getting around that. I just don't see what difference it makes if they knock. They've got a warrant. How does knocking make a difference?
Consitutionally, it doesn't. If you're a drug dealer with "product" needing disposal, it would make a BIG difference. :-)
"Unreasonable" in my mind is the grounds for the search found in the warrant and nothing more.
It keeps you from shooting them when they burst in and thereby, probably keeping you alive as well.