I was wrong. The mysterious sound was NOT being caused by the wind deflectors.
It was caused by my radio aerial.
With no wind deflectors on, I started my journey home from work but before I did, I moved the aerial so instead of being at a 45 degree angle, it was straight up.
I could hear the sound again, so remembering I had moved the aerial, I stopped and moved it back to a 45 degree angle and drove the remaining 18 miles home.
Not a sausage, no mysterious sound.
So, it was the aerial at fault. Luckily, no big repair bill for me this time!