Loud Vacuum Sound - 99 Chevy Sub C1500

[shultzee13]

There is a loud vacuum sound when turning off the motor like a valve closing. The loud vacuum sound is also audible when going down the road.

Strange. I cant seem to find it anywhere. Everything is connected like normal.

Gotta be something simple I am missing.
Thanks.

[69gmcc10]

Food for thought:

I had a strange sound coming from the engine bay and we searched all over to try and find it. After a smoke test we font the gasket between the intake manifold and the head had a good size leak in it. We replaced the gaskets, problem solved and no more noise.

Just something to check.

[shultzee13]

I forgot to mention that I just had a brand new fuel pump and fuel filter installed. I took off the breather and the sound is coming from the direct top of the engine were the fuel electrical plug is for the spider injectors. It seems like the engine is just depressurizing. But it's loud. I hear it in the vehicle when I am driving. A sizzle kinda sound.

Doesn't seem normal to me.

[69gmcc10]

The intake manifold is a 2 piece, and the top section slides over the oval section where the fuel and the harness pug in. The o ring that seals the oval section to the top is probably leaking or there is a crack in the plastic top section or both.

Google: L31 intake diagrams and you can find exploded diagrams of what I am talking about