In order to see sparking/arcing, you need to either have considerable CURRENT flowing or a significant voltage difference between your RXTX and the voltmeter.
What is connected to the RXTX??
What is connected to the voltmeter?
You might have a significant RF pickup by an antenna (any nearby transmitters?), or a static electric pickup by an antenna (dust storms or wind or rain can build up a charge).
The leads on the meter could act as an antenna for some frequencies.
A switching supply that does NOT have a good earth ground on it can couple AC from the power line onto the 'GROUND' of the powersupply output.
When I run my finger over the metal grill on my laptop, I can feel a small 'buzz' from ac coupled through the power supply. There is no 'earth ground' on the AC wall plug. I am careful NOT to use the laptop while near an earth ground! A voltmeter shows ~5 v ac when I touch one of the leads to the grill and stretch the other out.
Try battery power on the TXRX.
