That ratcheting noise is the nylon drive gears jumping the teeth on the slide rack when the drag on the rack exceeds the spring tension on the drive gear. This is "normal" when the slide is "full in" and "full out" though if you over do it it will prematurely wear out or break the nylon gears.
As capucine stated, if this is happening while in transit; you may have a serious problem. the gears may already be worn to the point where normal drag on the slide makes it skip teeth, the slide may be out of rig, or your gears may be missing teeth.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW