Exhaust whistle when accelerating?

[SKT Customs]

Hey guys, so for a long time (2+ years) my truck has had a loud whistle/hiss sound when accelerating (it sounds exactly like a big turbo), and I figured it was the exhaust tips or manifolds leaking - but it’s not. Any other ideas?? I tried tracking down/fixing the issue by doing the following in order:
1) I had Carven muffler welded into the factory spot & all the welds look good on that.
2) I removed the factory ‘Y’ pipe in the rear and now have a single 2.75” pipe running from the muffler to behind the passenger tire. Everything is welded together, no clamps at all.
3) I just had both exhaust manifolds replaced (picked up the truck from the dealer today)
4) The other flange after the cats looks completely sealed and none of the bolts are broken.

I’m really confused at this point. I thought after essentially replacing the entire exhaust from the manifolds back to the end of the pipe that this whistle/hiss would be gone because I assumed it was an exhaust leak. It sounds exactly like a turbo lol. Doesn’t sound like it’s coming from the engine bay.

Anyone have other ideas what it could be? It’s driving me crazy

 

[Scram1500]

Do you have an aftermarket cold air intake? I put a K&N system on a crown vic several years ago and it had a terrible whistle. It wasn't the filter itself but something inside the intake tube, like maybe a burr on the inner wall. Very high pitched and sounded like it was coming from everywhere

 

[kdoublep]

I had a k&n on my 98 trans am and it sounded like that but I liked it. Sounded good with the functional ram air system.

 

[SKT Customs]

Do you have an aftermarket cold air intake? I put a K&N system on a crown vic several years ago and it had a terrible whistle. It wasn't the filter itself but something inside the intake tube, like maybe a burr on the inner wall. Very high pitched and sounded like it was coming from everywhere

No I don’t have a CAI, not even an aftermarket filter. I replaced the original filter with another OEM one 🫤

 

[SKT Customs]

I had a k&n on my 98 trans am and it sounded like that but I liked it. Sounded good with the functional ram air system.

I wouldn’t hate it if I knew it was from a CAI haha but since I can’t figure out where it’s coming from it bugs me! I have a stock intake, stock filter. With the windows up it’s hardly noticeable, but roll the windows down and you hear it. And therefore I know people outside can hear it and it’s embarrassing because it sounds like an exhaust leak to me haha

 

[Scram1500]

Try a new filter or remove it temporarily, could be the filter itself or something tapped in the air box like a leaf

 

[1BADRAMLIMITED]

I wouldn’t hate it if I knew it was from a CAI haha but since I can’t figure out where it’s coming from it bugs me! I have a stock intake, stock filter. With the windows up it’s hardly noticeable, but roll the windows down and you hear it. And therefore I know people outside can hear it and it’s embarrassing because it sounds like an exhaust leak to me haha

I have the same type of sound you are describing I took my truck in for service because I thought it was the infamous manifold leak and they couldn’t find anything all the way back to the tail pipes.

 

[CG Rob]

I read somewhere a while back about the drain holes in the resonators making some sort of noise when the factory muffler was replaced with a high-flow unit. Not sure if that helps your situation, as I assume that you probably ditched the resonators when you upgraded the exhaust system.

 

[JOzzz]

TBS and CAI did this on mine. I’ve considered going back to stock to quiet it back down. Sounds cool for normal driving but pulling trailers all day everyday, it gets old.


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[SKT Customs]

Try a new filter or remove it temporarily, could be the filter itself or something tapped in the air box like a leaf

I’ll try it today! Seems unlikely because I feel like I’d hear the whistle at the air box, not from what sounds like the exhaust pipe, but I’ll give it a shot!

 

[Wild Bill]

 

[Rick3478]

Or maybe one of these.
The Carmonica

 

[SKT Customs]

Try a new filter or remove it temporarily, could be the filter itself or something tapped in the air box like a leaf

Tried this today. No change unfortunately

 

[Scram1500]

Do you still have the carbon filter in the lid of the intake? It's also possible there is a leak around the intake manifold small enough that an engine light wouldn't pop up

 

[SKT Customs]

I spoke with my mechanic tonight and he brought up a good point - since I can’t find a single thing wrong with the exhaust or intake system, we think that the heads are probably warped a tiny bit so even though it’s got a new gasket and manifolds, there’s probably still a small gap for air to escape. At this point I’m fine leaving it alone. Later down the line if I ever do headers and get the truck tuned, I’ll try to machine the heads down flat.

 

[1BADRAMLIMITED]

I spoke with my mechanic tonight and he brought up a good point - since I can’t find a single thing wrong with the exhaust or intake system, we think that the heads are probably warped a tiny bit so even though it’s got a new gasket and manifolds, there’s probably still a small gap for air to escape. At this point I’m fine leaving it alone. Later down the line if I ever do headers and get the truck tuned, I’ll try to machine the heads down flat.

Sounds logical especially given the location you are hearing it from

 

[SKT Customs]

Sounds logical especially given the location you are hearing it from

Yeah as soon as he mentioned it a lightbulb went on in my head lol makes perfect sense honestly. Definitely annoying but I’m happy that I feel satisfied with the answer instead of feeling like I need to chase it down. The whistle/hiss is way quieter after getting the manifolds repaired so it’s really not a big deal to me now that I think I know what’s up