We present the digital musical instrument TouchNoise that is based on multitouch interaction with a particle system. It implements a novel interface concept for modulating noise spectra. Each particle represents a sine oscillator that moves through the two-dimensional frequency and stereo panning domain via Brownian motion. Its behavior can be affected by multitouch gestures allowing the shaping of the resulting sound in many different ways. Particles can be dragged, attracted, repelled, accentuated, and their autonomous behavior can be manipulated. In this paper we introduce the concepts behind this instrument, describe its implementation and discuss the sonic design space emerging from it.