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We think it has to do with idler or pitman arm. When we took it to a chevy dealer, they said they werent bad, but starting to wear. They had no idea about the noise. It happens mostly on hiway driving and seems to diminish when we let off the gas. It is a rapid "tat-a-tat" noise.
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Does your Avalanche have 4 wheel drive? If so does the noise seem worse if you turn in either direction? If it does I would suspect a CV axle joint going bad. Another possibility would be a differential output shaft bearing making the noise. Check those two and see if you can pinpoint the noise?