It is real hard to advise on something like this with out hearing the noise and pin pointing the location of the sound.
I would save up your money and go to the dealer for something like this.
It could very well be a number of things that gives a distinctive sound such as you are experiencing.
Idler pulleys and harmonic balancers can give a sound and act the way you are describing, a reputable dealer might be the way to go.
〉 Answered on Aug 31st, 2007 by Amanda J. Pierce, MBA "AJ", Certified Mechanic, Project Coordinator; Primavera Scheduler at VIGOR Alaska Industrial Shipyard & BizzM3ch Solutions