I found a blog entry dated December 27 2007 which describes the fox's sound thus:
It wasn't the usual fox calls, more a rapid 'tapping' sound as though
someone was gently hammering a stone. [and] it was more like very rapid footsteps or tapping, but apparently it's an aggressive sound commonly heard during the courtship season
On the same page the author cites this piece of information taken from Irish Peat Conservation Council site:
The other noises are made when the foxes are in contact. One is called
gekkering, or clikketing, a mechanical staccato noise which sounds
similar to the clicking noise made by a football supporters rattle
The spelling of "clikketing" might be erroneous, I believe it should spelled as clickerting
On a website called World animal Foundation I found this description, again the fox mating call is said to be gekkering
The alarm bark: This monosyllabic sound is made by an adult to warn
cubs of danger. From far away it sounds like a sharp bark, but at
closer range it resembles a muffled cough, like a football rattle or
a stick along a picket fence. Gekkering: This is a stuttering, throaty
noise made at aggressive encounters. It is most frequently heard in
the courting season, or when kits are at play
The earliest reference I found on Google Books dated 1980, entitled Biogeographica. The red fox printed in the Netherlands and edited by the German animal behavioral scientist Erik Zimen.
This might help explain the origins of the expression gekkering. It's a stab in the dark, but it could be borrowed from German. I leave that task to someone else to find out.
From Bioacoustics, The International Journal of Animal Sound, and its Recording, 1993, Volume 5, pp 1-31. A copy of which is availbale for download here. I found the following entry
Type 5: Ratchet calls (Figure 7)
This call type has been referred to
keckern (Tembrock 1957), Clickerting (Margoschis and Burrows 1978), clicketing (Macdonald 1987), or gekkering (Macdonald 1987).
Ratchet calls occurred most commonly as a burst with a highly variable
number of components
Keckern is German for to make angry noises [of an animal] zool.