Probably a dumb question
 

I have a 89 with a 5.8 and true dual not cats with 40 series flowmasters on my truck. And I can't hear it. When I press the gas I get a noise from the engine like air it's not a vacuum leak truck runs great I'm guessing cause its an older engine but I dunno any ideas.

Like it isn't loud? And you hear air under the hood? Is your engine pumping backwards? Hahaha kidding.
Air like a forced sucking sound, possibly EGR?
How's your air filter?

you probably have a manifold leak...i have the 351 5.8l and i have a manifold leak and it sounds like air coming out of the engine...i also have a flowmaster but i left the cat on it...i can hear it but its not as loud as i would like it

How are your ears?

Well I think my ears are find wife would disagree but o well. When your stopped at a red light next to a semi and he starts to go and you hear air blowing as he takes from the engine bout the best way I can describe it.

It's fine when I'm idling but when I give it gas it just air I cant tell if it's sucking in or blowing out but it's loud and I don't believe it's a vacuum leak or if it is I've never had 1 that sounded like this

Can't hear it With the hood up revving it a lil? Have you done anything to yer intake?

Im runnin a vortech 40 style muffler an they're way louder than the flowmaster 40s I have on my 97 351. If yer tips go straight out the back thats bout how it'll be...I used to have mine out the sides behind the tire an I could hear it...but rocks just destroy the tips.

I have them comin out the side. And I haven't done anything to the intake. And I have rev it up with the hood up but I can't tell I'm no mechanic

I had my first truck full open exhaust, like manifolds to gutted cat and that's it cut off right behind the cat. And had no sound. R u sure your not hearing your fan blowing air.

I'm thinking it something like that I've spoke with other folks that have had similar truck just didn't know if the was a way to stop it and having the 3 speed tranny proly doesn't help