Apart from Gianluca's explanation, this expression cannot be evaluated
i2 = Interpolation[
Table[RandomReal[NormalDistribution[1, Sqrt[w]]], 3*noisefreq]][(3*noisefreq - 1) t/3 + 1]
Interpolation
returns a function that takes a numerical value within its domain, and this is not a numerical value
(3*noisefreq - 1) t/3 + 1