I have a few batteries charging in the 12 volt chargers for the Parkside tools. I have the smaller 2ah batteries. They seem to be making a really high pitched ringing sound when they are charging. I have battery chargers for other power tools and they never seem to make this high pitched ringing sound like these tools do.
It happens for me too. I do not think it is anything to worry about. I have multiple different batteries and 3 chargers. The same sound happens with all of them, no matter the combination. This makes me think that it is just something that happens with the chargers for these tools and is not something that you should be worrying about.