I inherited an Eheim Classic 600 (2217) which has been running on my tank for about 7 months but has started to get really noisy. The filter was 20 years old when I got it (but had never actually been used). It isn't the gurgling noise when you get air in the filter and tipping it up doesn't release any air but it doesn't really sound like grinding either which is the other noise people seem to report. I have taken apart and cleaned the impeller (several times) but without any real change (it does seem quieter after a water change as the filter is getting going again and clearing air but it soon gets noisy again. It kind of sounds like an air pump / fridge noise. My first thought was just to get a new impeller and shaft as worst case scenario I will have spares. Any suggestions on what to look at?