WSAK8118 loud hiss in bat detector

Just completed. No obvious construction problems.
Using earbuds for smartphones in headphone jack.
Everything seems to work, both channels.
Able to hear (presumably) US from various sources like electronic devices, tuning works as expected, above signals tune to different positions of potentiometer.
Very loud hissing static like noise between radio stations. Unpleasantly loud when volume above 1/3 full.
Is the hiss just the way the kit is supposed to sound?
Is the hiss due to the earbuds (it’s the correct jack). Does the kit require headphones of a certain impedance (ohms)?
If not due to the above: any suggestions on how to fix this?
Thanks for any suggestions!
In the meantime, I’ll check all component placement and solder joints. Then get out the oscilloscope and find where the noise comes from. Maybe a malfunctioning IC?

Some hissing noise is normal behavior of this kit.
On this page there is some technical info and a link to listen the recorded output signal of this kit:

That seems to be about the same level of hiss for the corresponding volume control position. So it seems to be working properly. Thanks for your reply.

No problem. I hope you enjoy the kit.