[HotZ28]

It depends on what the noise is; a rattling sound, of a bearing sound. Have you checked the S/C oil level? If it is low, you may have a bearing problem. In addition, you can take the belt off of the pulley and check for back & forth motion to determine the condition of the coupler. BTW, a noise in the S/C has nothing to do with an engine running lean; in fact, is very hard to determine a lean condition by visual inspection of a used plug! Your 02 sensor controls fuel trim and that can be monitored with a scanner.