More accurately than saying "an expression of some type" (which makes it sound like an idiom or figure of speech), it's a simple verb conjugation.
騒ぐ (the base form of the verb, i.e. what would be listed in dictionaries) means "to make a commotion".
The -ている form in this case is progressive/continuous (i.e. equivalent to the English "is -ing")
騒いでいる = [They] are making a commotion.
Changing the いる to いた makes it past tense progressive.
騒いでいた = [They] were making a commotion.
(The sentence in question omits the subject, which is possible in Japanese, unlike English. To know who was making a commotion, you need to know the context.)