Hissing sound under load, Chevy 350.

[Leviticus]

Hey,

So I just started getting a hissing type sound when the truck is under load, and you get on it. I don't hear it when I drive it normal, but like when getting on the freeway, it sounds like it is sucking air somewhere.

Not sure if it is just the normal sound of the air cleaner sucking more air or if I have a vacuum leak somewhere, I didn't notice it before. I have checked the upper vacuum connections, brake booster, distributor line, PCV, modulator (Upper connection) etc and all seem to be tight.

Where to look next?

Thanks Guys.

[Wrenchbender Ret]

Vacuum leaks are more audible when idling. I think you may have a restricted exhaust &/or an exhaust leak.

[justincat]

also it could be the gasket right under your carb that connects to the manifold. Make sure your carb is not loose. Spray a little carb cleaner around the bottom of the carb with it idling and see if it canges the idle of it. Will your in there make sure all the little rubber plugs arent cracked around the carb. You can spray there to

[Jacfourteen]

Originally Posted by justincat

also it could be the gasket right under your carb that connects to the manifold. Make sure your carb is not loose. Spray a little carb cleaner around the bottom of the carb with it idling and see if it canges the idle of it. Will your in there make sure all the little rubber plugs arent cracked around the carb. You can spray there to

If it were a carb/intake gasket it would be louder at idle or low load. when you're on the gas the intake vacuum drops near zero. I would say check airfilter to see if it's dirty or if it has come loose. That's all I can think of...