is it -4 inhg?
if so you have an intake leak... so your air that you're pulling in is going out before the combustion chambers
(tech tip: it's not the compressor on the turbo that makes boost, it's the combusted gasses hitting the turbine that make boost
)
you should hear it wooshing tho and normally it would raise the idle
do a diagnostic and check the codes if you haven't done so already