Type the simple subject of From somewhere in the middle of the crowd rumbled a spontaneous demonstration?

[A spontaneous demonstration] rumbled from somewhere in the middle of the crowd.

Note: In this sentence the noun "demonstration" is the simple subject. This is arrived at by asking the verb rumbled a "what" question. Here though the Words "a" and "spontaneous" do modify the simple subject and are properly treated as a part of the full subject in general.