Do a test. Listen to your DX100 with no music player when you plug the charger in. If the hiss sounds like it's hissing at a higher frequency than when it's not plugged in, it's a minor fault and iBasso will repair it for you. I have something similar but I just can't bare to part with my DX100 to get it fixed because it's pretty minor and everything else sounds so amazing. I'll have to do it eventually.
Regardless, get in touch with iBasso. They're pretty good in regards to customer service.
And I hope you enjoy your DX100 once it's fixed. :)
Thanks for the suggestion. I have just tried but I cannot hear a significant difference when plugging in the charger. On the other hand the hiss from time to time slightly changes pitch and intensity.
I have written to iBasso about the problem and will see what they suggest. I must say that in spite of the hiss I am quite impressed by the sound of the DX100 and it would be definitely painful to send it back to iBasso for repair :)