You may have heard your gelding or stallion produce a strange hollow sound from the sheath area when he is trotting/cantering.  People have probably told you that this means he needs his sheath cleaning, I thought this was the case too for many years.  It has recently been discovered that this noise is produced when the horse is tense or also if a gelding/stallion is showing off to a rival, it is caused by air being forced out of the space in the sheath as the muscles in the surrounding area tense up.  This explains why the noise is heard one minute but then disappears the next!
After reading of this, I have since observed that my gelding does only make the noise when he tenses up, either being ridden or worked online, and as soon as he relaxes it goes.  So there you go, mystery solved!