Added another AC50 to my 20g long planted tank. It was noisy! Brand new. This went on for a day (grinding, rattling). After inspecting and reseating it with no success, I swapped the new impeller with the old impeller and viola...both are quiet now.
In addition for extra water movement and filtration, I am using the second HOB
for mixing DIY CO2
and as a backup in case the first one conks outs.