I purchased an RO filter here in Spain from http://www.asfilter.com/ which is doing an excellent job taking the TDS down from 1250 to 68ppm.

However when the accumulator tank is connected, on start-up the pump vibrates and makes noise. See this short video: http://tweakshop.net/ro1/

When the accumulator is disconnected the noise is gone: http://tweakshop.net/ro2/

The pressure switch is fine. It also makes the noise if I plug the pump in manually after some pressure drop. Only when the pressure is sufficiently low after about half a gallon the pump works fine a well.

Any suggestions?