In this kind of construction, "it" represents an understood concept. Wherever the speaker may be is too noisy.
In order to determine what "it" refers to in the sentence, you need to identify the antecedent. The antecedent is the noun or noun phrase that "it" is replacing or referring back to. In this case, the antecedent is "noise". The phrase "can't hear you" indicates that the speaker is unable to hear something due to the level of noise. Therefore, "it" refers to the noise.
