When I first drive my car I hear this weird rattling noise coming from under my hood but as I pick up speed it goes away but once I slow down and start driving again I hear the noise

 

Does the noise sound like a whine, a grind, or the guitar solo in November Rain?

From what you wrote, maybe I would guess something is loose and the vibrations at lower speeds are shaking it around but it's hard to guess. Could you give us more info?

 

If you only notice it at certain engine speeds, it could be a loose exhaust heat shield.

 

what engine, can you get a recording?

 

I think I have the same issue. I thought it was engine knock. But it goes away when I drive. I can't hear it by just revving the motor.

 

could be ls pistion slap

 

Thanks Craby. But it doesn't sound like it