Odd noise after service

[Volucris]

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I have an odd noise in my Silverado (2007 Classic 4wd, 4.8)

 

I took it to a chevy dealer to have it serviced (Transfer case, differentials, and transmission filter and fluid, plus the belts.  they also inspected the rear brakes (drum)).  Now, it has an odd "metalic hissing" noise that comes in through the back window.  It sounds like something changed in the exhaust system, but I sure can't figure out what.  All the fluid levels are right where they are supposed to be.  Brakes aren't rubbing.  When you are driving down the road, put it in nuetral, and shut the engine off - it is still there.

 

It's not wind noise, if you step on the gas it gets a bit louder, but letting off the gas doesn't stop it.  It doesn't change pitch with rpm or speed.  And it wasn't there before they worked on it.  It is annoying the crap out of me.

You can also hear it at idle if you are outside the car a listening by the gap between the bed and cab.  Of course the dealer can't find it but they do say they hear it.

 

Something changed while they were working on it.  What could they have changed that would cause this? 

 

I know diagnosing a noise over the internet is pretty near impossible to do.  I'm hoping someone will say, "Yeah, I had the same problem and it was xxxxx"