Received response from Microsoft Customer Service:
I hope this message finds you well. Finally, I received an update from best Microsoft Engineers. We confirmed that When audio is stopped you will hear 1-2 seconds of white noise followed by a low volume clicking noise. This occurs as the amplifier built into Surface Earbuds turns off. This does not effect the functionality of your Surface Earbuds and is expected. Replacement wont help either because this symptom will occur on all Earbuds.
Best regards,
Ricky
So it looks like they are hissing by design! That is not acceptable for a brand new product - especially given that my 4 year old Bose SoundSport Wireless and the new Google Pixel Buds do not have this issue. Clearly it is possible to design headphones without a constant hiss from the amp.
I am very disappointed and will be returning the product.
They seem to think its only when audio is started or stopped but listening at a low level the hiss is still audible. Surely that is not by design on headphones that are meant to be used when presenting to people (as shown in their advertising for this product). I hope we can get a better response than what you received from support. I will also reach out that way and link this thread.