Our heatslave developed a fault a month ago. It was squealing baddly and finally went to lock out. We had an engineer out who replaced a bearing in the fan (I presume) Everything was fine until today when it started making the same noise and has again locked out. Are the bearings prone to fail so quick or is there likely to be a more inherent problem?. I am considering whether its past its sell by date. It dates from 1998.