The present study investigates human response to spectral changes in automobile interior noise, which characteristically has strong low-frequency content and much less high-frequency content. Specifically, we determined the smallest increases and decreases in sound pressure level in each octave band of automobile interior noise that can be detected by typical vehicle occupants. Increments and decrements in 1 dB steps in each octave band with center frequencies from 63 Hz to 4 kHz were presented to a group of automobile users, using a two-alternative forced-choice procedure. Results indicate that smaller changes (2-3 dB) can be detected in the higher-frequency octave bands (2 and 4 kHz), while larger changes (5-7 dB) are required for detection in the lower-frequency octave bands (63 and 125 Hz). The difference thresholds do not appear to depend on overall sound pressure level in the range 60 to 75 dB(A).