If it is only congestion then you can try some ocean nasal spray. Put two drops into her nose twice daily. This will help to break down the mucus if there is any deep within the nose but it will also help to dislodge any dust particles that could be causing inflammation in the nasal passages.
There are numerous causes to this congestion sound which include nasal polyps, allergies, inflammation in the sinuses, dental infection, reverse sneezing, or possibly a mass within the sinuses.
A dental exam, cleaning, and xrays would not be a bad idea to be sure we aren't having any issues here. I however, am very suspicious that we aren't dealing with what is called a reverse sneeze.
You can read more on this here:
I will let you know that there are still a lot of theories but nothing proven as a specific cause. So far allergies and post nasal drip are consider the most common causes. It is harmless and actually tends to be more concerning to the owner then it is to the dog.
Please let me know if further assistance is needed.