Strange intermittent relay noise
 

Hello all.
From time to time, I notice a relay-like noise coming from the front passenger side, first I thought it was the turn-lights relay, but it isn't as it is not synchronous to them. It has approx. the same frequency (about 2times/second), and changes whenever I touch the brake pedal. I am puzzled, and it only appears from time to time, the last time I heard it was two weeks ago.

Does anyone know what it is?

Thanks!
1991 190E 2.3, 8v.