914UL Ignition Noise

It might help to share a bit more information about your situation.  How many hours have you flown before this started at your current configuration or since you made any changes to your set up?  (Wiring - including rerouting, regulator, radio, battery, etc.)  It is hard to imagine why the capacitor would make a difference on only one ignition but we are talking the magic and mysteries of electrons.

I would start by checking the HT leads and resistive end caps. If you disconnect them all from the spark plugs and then use a multimeter to measure the resistance from one plug socket to its pair connected to the same coil. The resistance should be 4.0 to 6.0 kohm for each end cap and 5.0 to 7.6 kohm for the coil secondary. The HT leads themselves are non-resistive being made from copper. So total resistance from one socket to its pair should be 9.0 to 19.6 kohms. I find that in practice they are usually about 15 kohms.

If the resistance is outside this range then there is a problem. It is not uncommon for the end caps to fail and there can often be a bad contact between the HT lead and the end cap. This can be fixed by cutting off 10mm of the wire and reattaching the plug cap.