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It is really hard to give an exact diagnosis without actually hearing this noise myself or having the opportunity to road-test the vehicle. I can offer some different thoughts and advice that may help to solve the issue. It's good that you've noticed this problem and realize something is wrong. It could be any of the following:
As you can see the list is long and we have a lot more possibilities that we could add to the list. The best thing for you to do is have a trained professional road-test the vehicle with you. You should point out exactly what the noise is you hear and let him/her make a diagnosis upon further examination. You did the right thing, you heard something and you're acting on it quickly enough to avoid further damage.