I have a 10" flex pipe on the crossover (between the manifolds, just before the Y) on my DOHC swap. It whistles like an old VW Beetle on acceleration. I have had the entire system on and off the car 3 times finding and fixing leaks. I even made plates to block off the manifolds at the heads, installed the whole system, plugged it off and pressurized it to 10 PSI. No leaks. I then removed and tested all of the individual components. No leaks. Then I pressurized the crossover and bent and twisted the flex joint as much as possible. No leaks.

I put it all back on the car and start it. Feel around everywhere with my hands. No leaks. Use a heater hose to my ear and listen all around like a doctor with a stethoscope. No leaks.

When I rev it up or accelerate under load it whistles. What is going on here? My theory is that the flex joint I have is made of convoluted tubing covered with a stainless steel braid. The inside isn't lined, which leads me to believe the gases rushing through are reverberating in the convolutions and causing the noise (like the weird sounds a swimming pool hose makes when you swing it around over your head).

Am thinking clearly on this? Do I need a higher quality flex joint with a smooth interior, or should I just put a hard piece in there? Can I do that without it cracking?

------------------
1986 SE Aero coupe.

3.4 DOHC swap is complete and running, now just have to finish the rest of the car...

Anybody?

Are you sure it is that pipe? I have the Fiero store cold air intake and that sure whistles from sucking the air in

Positive. I have the factory DOHC intake tube on a Fiero air filter can with no intake hose (open on the bottom.) It also makes the noise with on intake tube attached.

Sorry but I'm not an expert on flex tube, however the only options I could think of for you if it is that for sure, would be to either buy a higher quality one, buying a shorter one (10 inchs seems quite long) or as you said put a solid pipe in there. Maybe you can wrap some exhaust heat wrap on it too so it muffles it.

Update:

Turns out the flex pipe was the culprit. It had no lining, just the corrugated pipe shape on the inside. The exhaust gases were flowing fast enough to create a whistling sound that sounded like a leak, but was impossible to pinpoint. I guess the sound was actually coming out of the tailpipes, but it sounded like it was coming from the crossover or the connections at the manifolds.

Anyway, after taking the exhaust off and pressure testing it 4 times, then finally replacing the flex section with hard tube, the sound is gone. On to the next thing.

Moral of the story? Do not use a low quality flex pipe. This was an ROL brand from Oreilly's, 10" long. Maybe it was too long? The crossover that was on the engine originally had a flex section with a smooth liner. I guess GM already went through this exercise. I screwed it up trying to make it fit, so I ended up with this mess.