Weird Sound at startup 2015 Sierra Denali

toro1966 · August 23, 2015

Hey guys - I still have the new guy OCD - notice everything with my truck - syndrome.

One of the things I noticed is that - especially when cold or first started - there seems to be a weird (technical term) noise just after she starts up. I would liken it almost to a starter or motor engagement. It is fairly quick and goes away right away. Again - it's just after the motor starts.

On subsequent starts, it really doesn't do it. Weird. Again - this might be perfectly normal, just wondering if others have noticed it. Thanks!

fondupot · August 23, 2015

Like this? If so, it's the exhaust passing through the flapper valve. It's normal.

toro1966 · August 23, 2015

Thanks! Exactly that! Knew I wasn't crazy - well at least not about this Thanks again!

20SierraL8T24 · August 24, 2015

Both of my 14's did this. Nothing to worry about I assume.

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BuyAmericanIron · August 24, 2015

I read somewhere that there is an auxiliary pump in the 8L90e (8 speed) transmission that primes upon starting after sitting for several hours. When my truck does this it actually moves a bit even though it is still in park. It is different than any vehicle I have ever owned, but seems to be normal. On my second transmission and both have done it so I am guessing it is considered a normal sound.

fondupot · August 24, 2015

I read somewhere that there is an auxiliary pump in the 8L90e (8 speed) transmission that primes upon starting after sitting for several hours. When my truck does this it actually moves a bit even though it is still in park. It is different than any vehicle I have ever owned, but seems to be normal. On my second transmission and both have done it so I am guessing it is considered a normal sound.

It was also in trucks with 6 speed transmissions.

I dont hear it anymore with my aftermarket exhaust. So I think its definitely the flapper valve.