Welcome, my name is XXXXX XXXXX X will do my best to help you with your issue.It could be low gas pressure. Gas pressure that's outside of the acceptable range can cause a burner to whistle or howl occasionally. The pressure may not be so low the unit won't function but just low enough to cause it to make noise. This is more likely in very cold weather when demand is highest
Very cold weather can also affect your propane regulator which could impact supply pressure
The run isn't that long, ½" tubing less than ten feet. Would it be wise to have the regulator checked or replaced?
You might want to check the supply pressure. It should be around 11" WC (water column) If the regulator setting is off then you should be able to adjust it or replace it if it doesn't respond. It's also a very good idea to be sure the regulator is sheltered from the rain and the element so no water can affect it. This isn't really that significant a problem (since you say the unit functions normally) unless you find the noise annoying.