Reply to Deck A Cue Clicking Problem

A couple weeks ago I was doing a live set when I started hearing a clicking in my headphones whenever I cued Deck A. Luckily the clicking only shows up in the headphones and not in the mix but I find it strange and can't figure out what causes it. I can describe the clicking sound as a single "click" on every downbeat. When I spun a set last friday I didn't hear it for the first half hour but for every song loaded and cued in Deck A after that. Any ideas?