Geldings make noise when they run primarily due to the tensing of their abdominal muscles, which causes air to be sucked into the sheath. When they extend their stride during running or trotting, pressure is created as their abdominal muscles elongate, and this leads to the characteristic "whooshing" or groaning sound. This noise is normal and is not related to testosterone levels or dirtiness of the sheath, although sheath cleanliness can sometimes affect the sound's presence. It is often heard at faster gaits like the trot or canter and can vary between individual horses. The sound results from the interaction of muscle tension and air trapped in the sheath area, not from any health problem in most cases.
