You could be hearing a roger beep (which is misnamed -- even though everyone calls it a "roger" beep, it should be called an "over" beep, because "roger" means "I received everything you just sent", while "over" means "now it's your turn to talk"). That's the generic name for a beep that your radio sends when you un-key the microphone. Those tones are very common on FRS radios, but not so much on ham gear. Some Chinese radios have the capability, though. Around here, that kind of beep will bring people out of the woodwork faster than a CQ call. But some repeaters have a little "courtesy beep" when the repeater itself stops transmitting. I'm guessing that's what you're hearing. This would be a brief beep that happens just before your S-meter drops to zero. It's an indication that the repeater has gone completely quiet and the repeater time-out timer has reset, so it's safe to talk now for as much of a long-winded transmission as the repeater is set up to allow.