Strange ticking noise from the play head

[Kobra]

OK, so I am a happy camper for the most part, no issues in general, but this is a head-scratcher:

Patched in an external mic pre on Analog 7 to track some vocals here and got some ticking noise on the recorded track. After investigating a bit, it turns out that it is the play head ticking along in the Edit window... just like that scrolling sound effect of the Mighty Mouse.

The work-around to get rid of this was to create a screen-set with the mix window only. A bit inconvenient when recording, since I like to watch the Edit window to make sure all is well while recording / to set the in and out points etc... not to mention the pretty vibe-y vocal takes that had to be scrapped .

I am aware that using an unbalanced cable going from the mic-pre (which has unbalanced outs) would be subject to interference, but it's still puzzling that the noise would be coming from the play head... or is it not?