HI, There are a few thinks that can cause this problem (depending on model) there is a little red and black coloured bottle shaped component in the elbow of the outlet tube, This can cause pulsing Also there is a spill valve this could be siezed and stuck in the same position this causes the same problem.
Hope this helps, Any parts required can be obtained from
www.candmdomesticappliances.co.uk
paul
April 2009