A bad belt tensionier will cause a squealing sound because the belt is slipping on the pulley. What you described sounds like something is loose in the front end, or the exhaust system.
To start, take a rubber mallot and tap on the exhaust system to see if you can hear the sound. Many times heat shields or the inside of the catylatic converter can make the noise you described.
Let me know what you find. Also, when you get back to me, be as specific as you can when describing what you hear and when it happens.
Joe
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Tuesday, October 13th, 2009 AT 9:13 PM