Hi   -how old is your filter? I'm wondering if there is something worn or broken? As I only have xperience of internal canister filters I can't give you a definitive answer but can offer some suggestions so apologies if anything I mention is incorrect, I'm just guessing here!

Make sure there is not any air getting into contact with the impeller-air is very harsh on filer systems & will make a lot of noise-I have a couple of canister filters & if I don't prime them properly after I switch them back on a lot of air is trapped inside & forced against the impeller which makes a terrible racket!

another possibility is like the above poster said-check for sand residue if appropriate & if the working parts are ceramic there may be some surface scratching that is making the noise. You may find the spindle/shaft/axle for the impeller is made from ceramic material & is broken-or not sitting true in it's housings?

If you cannot correct the problem you will have to get yourself anew one but don't turn the old one off until you have the new one. It is vital to maintain filtration constantly for Goldfish. You can put the established media from your old one into the new one so there will be continuity of the cycle. If it is bigger then the old material will help seed the new.