Torque Converter Hiss
I've been wondering what this is for a long time and today after driving around with the windows down got a bit annoyed. I just figured I'd ask in case it turns out to be something that eventually hurts the TC.
Basically I run H&S S2L which is 5th lockup only @ 45mph. When the TC locks, there is a loud swoosh or hissing noise similar to a boost depressurizing noise, though much slower. Hard to describe but if I'm just cruising around with minimal throttle (no boost) and the TC locks, as the RPMS dip there is a sppshhh noise. I can manipulate the sounds with throttle (like it's building pressure), say if I give it some throttle while it's pshhing I can carry it.
It could be coming from the turbos but it's really loud and sounds like it's coming from under the truck. It also doesn't stay in sync with boost pressure (gauge) so not sure. Doesn't sound like any of the typical turbo noise I hear outside of TC lock.