You're on the way to gettings things in order-- there are likely a bunch of adjustment issues to be tweaked. Good Tech Day Fodder.
That "death rattle" is called running-on or dieseling. Caused by ignition timing or mixture issues, low octane fuel, a hot spot in the combustion chamber or simply a chortle of joy that the carbs are better.
Used to be a very common problem in the early 1970's when (then new) engine emission controls and/or lower quality gas came to pass.
'73 R60/5 Toaster