Vehicle in question: 2004 Subaru WRX wagon.
This weekend, I replaced the intercooler hose (y-shaped upset the intercooler), the throttle body hose and the blowoff valve hose (routes the over boost back to the intake when the throttle closes suddenly). New hoses are silicone, thicker than stock and have significantly smoother curves. Plus, they're red so that should add 0hp per hose, right?
Install went pretty well: old hoses are tired, have plenty of grease and oil accumulated after 12 years and are ugly. The new hoses are thicker and come with super robust clamps. One of the clamps was a bit too small (blow off valve end of the route-back hose) so I had to reuse one of the old clamps. (Pro tip from future me: think harder about the clamp rotation next time!)
The only problem that I'm left with is a high pitched tone that triggers at nearly peak boost. It sounds much like a balloon with a hole in it and only manifests under load and under a lot of boost (e.g., going up a bridge in high gear at full throttle). It isn't subtle which implies that something is venting to atmosphere.
So, what does this sound like? If it were a bad gasket, I'd expect to hear a rising tone over time. Same if it were a hose clamp or a loose hose. It could be a blow off valve noise but I'm super surprised to hear this much racket: the stock blow off valve has always been very subtle (no big whoosh on drop throttles for instance).