I'm having the same issue on my 18. The sound comes from the right side, I say it sounds like a weedwhacker or Honda Civic. Had it back at the dealer twice and they tried to say it was a vacuum hose which I had already replaced due to that horrible grinding sound it made. After 2 days at KG Ford, they gave it back to me saying it was fixed. Soon as I stepped on the gas, weedwhacker was back. The sound goes away when I get to around 40 mph. When I don't want to hear it I take off slow and get up to speed without the noise.