Ok if its clicking but no visable spark then what it may be is a faulty spark lead.
As what can happen on gas hobs is a spark lead my wear under the hob so what happens is it shorts out against the metal work of the casing under the hob and causes the sparking.
If you can turn the power supply to the hob and see if the clicking stops. If it does then its either one of the spark leads at fault or the spark generator that creates the spark is the problem. As it can only be them that can cause a clicking noise in there.
If you are looking at fixing this yourself you need to remove all the burners on the hob and the control knobs.
Then remove the all the screws around each burner and then you can lift the hob top off.
From there plug it in and very carefully watch it and see if any of the white spark leads are sparking under there. If so then you need to replace the faulty lead.
But if the leads are all ok listen too see if the noise is coming from the spark box and if so then that will need to be replaced.
Check them out and see what you find. But for a clicking noise its usually either the spark lead or the spark generator that causes this.
If you need anymore info just get back to me.