What would be wrong with the waste gate?? If it's sticking open, I wouldn't make any power, if it's sticking closed it would over boost and I would think it would either trip a check engine light, or have some fail safe that would shut the engine down??? The truck isn't physically surging like boost is going up and down, it just overall feels like a lack of power when this is happening.
This noise is definitely coming from the air intake area. The waste gate is all on the exhaust side.
I don't know though. I have a lot of experience with turbo'd cars, but none with diesels, so they might be setup differently.