tdi 300 buzzing?

hi all i own a 300tdi es, every now and again when i turn the disco on theres a buzzing from under the bonnet and even if i turn the ignition off the buzzing continues. this bussing is always accompanied by the abs light staying on ,but the light goes out when ignition is turned off but the buzzing stays. under the bonnet i found the source of the buzzing is the abs pump? right hand side as you look into the engine bay. if i pull the plug on the pump the buzzing stops and if i leave it for a while and then plug it back in everything is fine for a few weeks any ideas? any help will be appreciated. cheers all