I think I see some confusion here.
I dont think it's legal for the police to enter a private place based upon probable cause. They may enter with a warant. That warrant must be based upon probably cause. That probable cause must pass a two pronged test, and be aproved by a justice, or a judge.
As I remember there are several exceptions to this rule
1) Permission.
2) Exigent circumstance
3) Danger of loss or destruction of evidence.
4) Evidence open to plain view
There may be another my old mind cant remember... forgive me.
In order to obtain a no knock endorsement the judge must agree that knocking, and announcing will put the officers lives, or the lives of others in danger.
It seems that some judges are granting no knock warrants as a matter of course for anything involving narcotics. I personally think thats bad.
There's also some confusion about how these warrants are served. What we mostly see on TV and all the swat show's isnt a dynamic entry. It's a controlled entry. A SWAT team stacked up, flash bang's, armored vehicals, house surrounded, a plan in place, intelligence in place... All of this add's up to a controlled entry. This is and should be the method in 99% of all narcotics warrants. It's also the safest method for the police, and for the subjects in the warranted location.
A dynamic entry is usually less organized, and is more critical situation driven. Traditionaly a dynamic entry is used when there isnt time to organize a controlled entry.
A Controlled entry is ALWAYS conducted by SWAT or a similar unit that has been trained, and experienced in this work. They are equiped with the proper tools, and again experience for this. When conducting a controlled entry they have been briefed, set up a plan, a disaster plan, assingned duties to each member. They are a team working by the same rules, and plans. In my experience they are 99% successfull.
A dynamic entry could be performed by a SWAT team... but could be performed by other properly trained officers. The best way I can describe a dynamic entry is by saying it's an adjustable entry. The officers must adjust to conditions as they develop.