Hi . I replaced my wilo smart heating pump today as it had developed a problem with surging . I fitted a grundfos alpha2L 15-60. The pump is preset to pp2 and is recommended for most systems . On first start up their was a chattering from the pump which I first thought was air . The noise eventually went but comes and goes intermittently . The only way I can stop the noise perm is to turn it down to pp1 but don't want to cause other problems by doing this . I have googled the problem and sorry know nothing about flow rates etc which all the info refers to . I have an open vented system with 12 rads and a new boiler fitted in Nov 2015 . It also has a manual bypass . Any advice would be appreciated