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So, my daily driver 04 Silverado 1500 Z71 (4.8l) is making a buzzing / humming noise while shifting, but only for the first few minutes of driving. The noise is strange, sounds almost like a dying plastic window fan. I am not a super-mechanic, but I know my way around a car, and this has thrown a curveball at me. Idk what this noise could be. After the 2 - 5 minutes of making this noise during shifting it stops and the truck drives/shifts perfectly and quietly.

Im baffled. Ive asked around and gotten answers that range from "overdrive kicking on" to "trans filter clogged" to something about a "high pressure valve". Just wondering if anyone has come across this in the past.

Oh, just for reference it does not "clunk" while shifting like everyone talks about.

Thanks for any help in advance
 

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Could be the transmission fluid maybe? I say that since you say the problem occurs during the first few minutes of driving, before the transmission and other fluids have come up to temperature. When was the last time you had your transmission fluid changed?
 
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