Chasing down an intermittent P0171 code, did the suggested Vacuum leak check using the PVC tubing airbox adaptor and 12 psi of pressure. Found a melted check valve by the airbox, replaced same. Now when pressurized I get a 'hiss' I can't put an exact finger on back by the combi valve (fairly certain it's not the short piece of rubber tubing attatched to it). Plus, a gurgle is heard down in the oil pan. Related? Bad combi valve? Suggestions?

I just did a boost leak test too. Im most likely sure that the gurgle is coming from the turbo oil return line, which means a seal is possibly shot on the turbo, but not 100% sure

Well, three days, multiple hot / cold starts, and no CEL, so hopefully it was the check valve I replaced. I think I'll just let sleeping dogs lie.

Put my mind to other things like a boost gauge and a catch can.