1. A method of determining an estimate for a noise-reduced value representing a portion of a noise-reduced speech signal, the method comprising: generating frames of an alternative sensor signal using an alternative sensor other than an air conduction microphone; generating frames of an air conduction microphone signal; identifying frames of the alternative sensor signal that contain speech; determining whether a frame of the alternative sensor signal that contains speech is corrupted by transient noise based in part on a frame of the air conduction microphone signal, wherein the transient noise is detected more by the alternative sensor than by the air conduction microphone by determining a value F t and comparing the value F t to a threshold value, where the value F t is determined as: F t = ∑ k = 1 K ⁢  B t - HY t  2 σ w 2 + σ v 2 ⁢  H  2 + σ H 2 ⁢  Y t  2 where K is the number of frequency components in the frequency domain values of the frame of the alternative sensor signal B t and the frame of the air conduction microphone signal Y t , H is a channel response for a path from a speaker to the alternative sensor, σ w 2 is a variance for sensor noise of the alternative sensor, σ v 2 is variance for ambient noise and σ H 2 is the variance of a prior model for the channel response H; and estimating the noise-reduced value based on the frame of the alternative sensor signal if the frame of the alternative sensor signal is determined to not be corrupted by transient noise.