This mightnt be the best way to word it but stick your fingers in a random fan. If the sound continues, its not that fan.
Of course, I dont literally mean shoving your fingers in there. If you dont end up with a nice cut or 2, you could always damage the fan and cause a new problem. I put my finger on the hub of the fan gently to slow the fan down slowly.
Once fans have been ruled out, disconnect any culprits one by one to find it.