Many of the Kemo products are available with DC power input for portable applications. Typically 9 - 30 V DC power input, where 12 V or 24 V DC is available from batteries or a vehicle supply. 
Most DC powered units use DC - DC converters, these are switching devices and they generate high frequency switching energy. Depending on the devices used and their type the level of this energy varies, but it can be several volts on a typical unsmoothed 15 volt supply. The energy is typically a high speed switching pulse of several MHz frequency content which repeats at about 100 kHz. This energy will get into most signal circuits as some of the content is radiated.
We use the best practice possible in our circuit board layouts, wiring, and power filtering around the DC-DC converters. However some of this energy still gets into the signal paths, and is seen as high frequency 'noise'.