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Start with the simple things. Inspect the accessory belt AND the tensioner. Remove the belt and inspect both sides for glazing, obvious wear, and contamination. Start the engine without the belt to determine if the noise is still present. If not, either the belt, tensioner/idler, or one of the power accessories is a likely suspect. Since the frequency of the noise is relatively constant at any RPM, it is more likely belt slip than an accessory/bearing failing.