A number of months ago I had H&R lowering springs and spacers installed on my 2011 V6.  I also had to have an adjustable panhard bar installed.  At some point after that, I began to hear an annoying "squeak" coming from the right rear area.  Very loud at times but not constant.  It primarily happens on the freeway as the car is moving over the gradual up and down of the pavement.  I hardly ever hear it on residential streets.  I've had the car back to the shop several times and they've spent countless hours trouble shooting this and looking at every moving part that could be making the noise in the right rear.  What could be tightened was and they even put some Accumat in areas they thought could be rubbing.  Nothing has worked.  The squeak is still there.  The shop has run out of ideas and I'm at my wits end putting up with the noise.  Has anyone ever heard of this?  Does anyone have any thoughts on what it could be? 

I bought a 2011 Mustang V6 last October and have had some custom work done, including:  after market sound system with navigation, satellite radio, all new speakers (including two trunk mounted 10" subs), hood struts, cold air intake, lowering springs, wheel spacers, Corsa exhaust pipes, HID head and fog lamps, car alarm and window tinting.

The next two items on my wish list are a supercharger and stainless steel brake lines.  Any advice or suggestions on those two items would be most appreciated.  Thanks!