It is possible that the whistling sound you are experiencing while lying down is due to post-nasal drip, where mucus from your sinuses drips down into your throat. To help alleviate this, you can try using a saline nasal solution to clean out your nasal passages. It is also important to stay hydrated by drinking plenty of fluids. If the problem persists, it would be best to consult a doctor for further evaluation and treatment.