FWIW, If you can't identify a rub causing the howling sound, it may be the infamous 'moose'. This is caused by a resonance in the air intake and sounds like, well, I guess a moose groaning. There is a TH fix listed in the how-to section that typically clears this sound. The moose usually shows up as the weather gets colder. I think I cleared mine, but wont know for sure for another month or so...
