I had the same problem with the whistle only in the rain. There are a few posts about the whistle under normal driving but non during rain. Any way I never found out why or how come it happens only in the rain. Some thought it might be the wheel well causing the problem. So I ended up following the thoughts of the whistle all the time folks. That required getting a different grill from Thor. I called Thor customer service up and trying to explain to them what happens (which was hard) they said they would ship me the replacement grill.
Once the grill arrived I was going to take the original one off and replace with the new style. I realized that I really did not want to start that project. So I took duck tape and taped the edges of the new grill, screwed it (using original grill screws) to the back of the original grill.
Result that during rain my whistle factor has been cut down to almost nothing. I do find alittle whistle around the 50-55 speed but nothing like I used to. Good Luck
Retired and never looked back