Is this normal?

[BatKing]

Hi everyone,

My 2004 X5 4.4 is doing some funny sound when cold start.

When the X is cold (such as in the morning), I start the engine, it is loud but then after 30 second to 1 minute, it will be quiet. However it is giving some rough idle sound.

such as "Woo ... Woo .. Woo..". it is like the RMP is not stable and the engine is running up and down and up. but when I look at the techno, the RMP is fine. and if I open the hood and listen to the engine, the engine is smooth. the sound is not from the engine but from the back of the engine...

Then, if I get to the back of my X and listen from the exhaust, it is making the "Woo... Woo... Woo.." sound too.

Is this normal? If the engine is hot and I start it, i can't hear those funny unstable sound.