Purpose of this second cap on Pastyface power input?

[EBRAddict]

What is the purpose of film capacitor C9 (circled in red) on the Pastyface power input? I understand what everything else does (mostly), and C9 isn't part of the basic inverter layout in the data sheet. I searched but didn't see anything.

[midwayfair]

film caps have lower ESR and are better at filtering high frequencies than electrolytic capacitors. It makes the power filtering better.

[EBRAddict]

Thanks! I'm getting a tinnitus-grade squeal on the audio path when using the inverter vs. a battery so I wanted to figure out what everything did.

ETA: I see I didn't connect pin 1 per the schematic (I'm on a breadboard). Leaving it open = 10kHz switching frequency (tinnitus!), putting it to V+ sets the switching frequency to 45kHz, which coincidentally is just above the sampling frequency of many ADC.