I had a cold around three weeks ago, since then I've had a productive cough. The phlegm is thick & sticky but clear/white. In the past few days I've been getting a strange 'vibrating' noise upon inhaling and exhaling and more so on deep breaths. If I cough, I remove some phlegm and the noise sometimes stops. PF is ~350 from 550 but I don't feel that symptomatic. Any suggestions what this could be?