Many times the crackling sounds may be from the throat or bronchus (upper part of wind pipe ). When baby coughs these sounds become more evident.
If there is really fluid in lungs (like pneumonia) there will be some signs- baby breathing fast or baby finding it difficult to breathe , taking less food or refusing food altogether, baby dull and not playing etc.
Body's immunity protects from bacteria / viruses damaging lungs.
Next Steps
If you think baby has any of these warning signs, you may meet your pediatrician again and discuss with him in detail.