We had the same thing, I called Thor and they have a replacement grill that has the honeycomb shape. If you have the grill that has small squares that is probably why it whistles. Once my grill was replaced there was no more whistling.
Dave, U.S.A.F. (Ret)
2019 Fleetwood Discovery LXE 40M (Big D)
2016 Ford Focus, flat tow
We just returned from a two week road trip and had the experience of driving through coastal rain where the whistling would start as soon as the rain began and end as soon as it stopped. I thought i was going crazy but figured I'd give it a try and search for the term "whistle" in the forums. Imagine my surprise (and relief to tell you the truth) to know that this is a known issue. My 2018 34J has the square holes in the grill.
Question - is it a grill of some mesh addition? I called and was told they'd send me mesh.