If you decide to observe and/or record impedance values together with the EEG signal, there will be high frequency noise.
It stems from impedance measurement, and is embedded in the recorded signal. This noise is at the frequency of 125 Hz for 500 Hz mode and on the 62.5 Hz for 250 Hz sampling frequency. This noise can be later filtered out using a notch filter. Also, the input referred noise in the impedance recording mode is higher, as the input impedance of the amplifier is decreased from 1 G-Ohms to 500 M-Ohms.
So, we advise to turn off impedance monitoring during recordings.
High-frequency noise on 50Hz/60Hz (depending on where you are) can be due to poor electrode contacts. This would be indicated by high impedance values. To reduce this noise, ensure better contact on electrodes/reference electrode.