Periodic noise during use of the Ganglion Board

Hello everyone,
I am a new user approaching the world of bio-signal acquisition. More specifically, I recently purchased the EXG kit, which includes the Ganglion Board and all the necessary components for its use (electrodes, usb dongle, battery, etc.). A few days ago I started using it, following the guides available on the OpenBCI website. My final goal is to study EEG signals recorded from the primary motor cortex. However, before reaching that point, I decided to proceed step by step. I have already performed the following recordings (following the official guides) without any issues:
1) ECG, using two input electrodes on +1 and -1, and the reference on G_D, with a right arm–left arm–left leg placement. I used gel electrodes and snap cables.

2) EMG, using two input electrodes on +1 and -1, and the reference on G_D, placed on the proximal–distal forearm, with the reference on the elbow. I also used gel electrodes and snap cables.

3) EEG, using two input electrodes at Fp1 and Fp2, with reference on the earlobes, connected to REF and G_D. I used dry polymer Ag/AgCl electrodes (the ones available on the OpenBCI website), ear-clip reference electrodes, snap cables, and a headband to secure the reference electrodes.

However, when moving to the next step of recording signals from the primary motor cortex, I encountered a problem: a strange noise appears constantly on all four channels. After connecting two dry electrodes to +1 and +2, and the ear-clip electrodes to G_D and REF, the recording started showing a continuous periodic noise (so much so that at first I thought it was an ECG artifact). I tried changing several times the placement of both the recording electrodes (moving them to Fp) and the reference electrodes (placing two gel electrodes on the mastoids), but the noise never disappeared. The input impedances appeared to be within normal range, but what confused me the most is that even when only the reference electrodes are connected, and even when they are not attached to the body, the same periodic signal is still present in all recording conditions (I also tried ECG and EMG recordings, and the same issue appears).

What could this be due to? How can I fix it? Am I doing something wrong? I am also attaching pictures of both successful and failed recordings.