Breathiness is an aspect of voice quality that is difficult to analyze and synthesize, especially since its periodic and noise components are typically overlapping in frequency. The decomposition and manipulation of these two components is of importance in a variety of speech application areas such as text-to-speech synthesis, speech encoding, and clinical assessment of disordered voices. This paper first investigates the perceptual relevance of a speech production model that assumes the speech noise component is modulated by the glottal airflow waveform. After verifying the importance of noise modulation in breathy vowels, we use the modulation model to address the particular problem of pitch modification of this signal class. Using a decomposition method referred to as pitch-scaled harmonic filtering to extract the additive noise component, we introduce a pitch modification algorithm that explicitly modifies the modulation characteristic of this noise component. The approach applies envelope shaping to the noise source that is derived from the inverse-filtered noise component. Modification examples using synthetic and real breathy vowels indicate promising performance with spectrally-overlapping periodic and noise components.