fluid in left lung

my husband just had fluid drained in his left lung. they inserted a tube through the back,fluid color looked reddish,yellowist,doctors are saying they don't really know the cause of the fluid.  My husband has like a dry cough.
He has a difficult time breathing,especially when cold air or draft hits his face.  He is still in the hospital.  When he starts to talk,he'll start coughing and then he has a hard time breathing.  Please help!.  Cardiologist came in this morning and said that he hears a crackling in his left lung again and they just drained the fluid out like 4 days ago.  can anyone,please,tell me what this is or what is causing the fluid in his left lung and why is he coughing whenever he starts to talk and he also has a small amount of fluid in his heart.  Desperately searching for answers.  Thank You.

Long Island,NY