Vauxhall Zafira heater blower very noisy and not working efficiently

For the past 18 months the heater blower has made a droning noise particularly noticeable at settings 3 and 4. It does progressively get noisier from settings 1 to 4 but 3 and 4 see little extra air circulation.This all follows a service but have not resolved. Have removed pollen filter and run without this as a test but stilll noisy. Garage suggested could be a bearing or similar on blower. Any ideas and if I buy a replacement blower then how easy would this be to fit ? Thanks