H300 ampl. noise when charging

[lwizard]

Hi.
I have an H340 with rocbox 3.2.
I use it in a car, often leaving the iriver conected to car adapter.

Sometimes, probably when the battery is fully charged, the iriver starts to output a disturbing noise in the music,
like an hisss.

It become worst when the backlight turn off, disconnecting the power adater the noise goes off,
if i reconnect it, the noise reappear after some time (some minutes).

Is it possible to optimize power menagement to reduce interference with the amplifing section of the player? thanks!