If it's an analog voice channel on a Motorola (not P25) trunked system, or if it's a conventional channel that isn't set to "CTCSS" or "DCS", it's normal. The scanner is "waiting for possible digital", where the default wait time is 600ms.
You can change the default wait time via the scanner's PGM FUNC GLOB menu, under the item "DG Int Prime". I have mine set to 30 (300ms).
EDIT: Some caveats when changing the value from the default:
- The likelihood of "digital motorboating" increases as you decrease "DG Int Prime". If you're not monitoring any P25 digital voice on Mot3600 systems, this isn't a problem.
- The chance of missing the start of analog voice increases as you increase "DG Int Prime".
- You can mitigate the effects of the above by a) programming any always-analog CONV objects in CTCSS or DCS mode and b) programming any always-digital CONV objects in NAC mode.