Is your ro tank full? My solenoid makes a noise described like that sometimes when my ro trashcan is full and I get back pressure from the unit. I try not leave the water on to my RO unit, just long enough to fill my can, then cut it off, back on when the ato sensor says its low.
I figured it out. I disconnected the red line that hooked up the the water supply in the wall. That seems to let all the air out thus reducing the vibration significantly. Thanks!