2007 Dodge Nitro Vaccum sound in dash or firewall when I AC

I recently put JBA headers in my truck but when I accelerate at certain RPMS I hear a vaccum sound in
the dash or firewall, could this be something that
the tech left loose or maybe a loose vaccum line.
Recently the check engine light came on and went to
the dealer and they could not pin point the sound
any idea what this could be?