It sounds to me like it's your receiver. Scanners are notorious for receiving signals which are not on the frequency to which they are tuned. This can occur for a variety of reasons and in your case 121.5 maybe being interfered with by another signal. For example, you may have a nearby transmitter - police or.t other sevrices - which are breaking through although you may not hear any voices. If the noise is continuous it may be a paging service or similar.
Do remember the law about using such equipment.