I switched mine out for a MJ 1200 for more flow, but I still use the original to mix salt in my office, and it makes no appreciable noise. I would take it apart and examine the impeller shaft (broken shafts are a common cause of impeller noise from rattling).
That said, many IM 20 owners replace the stock pump for either a MJ 1200 (Cobalt brand, not Marineland) or Sicce Syncra 1.5 to gain additional flow, and either of those will be relatively noise-free.