Advice on powerheads

[Frenchie2]

I have 2 Hydor Koralia 4 (for almost 1 1/2yrs) and they are both "rumbling" like a high vibrating sound that would drive you nuts. I phoned J & L and they thought it might be the impeller. Nobody carries this particular part that I know of, so I ordered them from Aquariumparts.com. Changed the impellers, cleaned the powerheads, put them back together securely and tested them only to find out one of them is still making that rumbling sound (if I was a fish or a coral - I would go insane!). What do you think it could be? Should I get new one(s)? Please help. Thanks!