I have a Vauxhall Vectra CDTI 150 2005 model. I have a problem with the power steering pump, which works intermittently. On start up it either works or it doesn't, and when it doesn't work the pump (electric with a white bottle on top) will knock a bit, like a heartbeat. I have so far changed the battery and alternator and also the pump itself, but no cure. I also had an auto electrician look at it. He plugged it into his diagnostic machine but reported nothing, and he himself was baffled. Have you any ideas?