I'd usually side with Bubb Rubb but I had a whistle under boost that started last "Winter" that I thought might be a tiny leak somewhere with my PCV hose to TIP connection somewhere, and then thought I had made it "go away". Well it was a freezing cold 54F here in South Carolina the other morning and what do you know, the whistle under boost is BACK. OP is your car bone stock? Was it colder than it had been during your morning drive?