just came across this post and I had to laugh.
Laugh because I had a very similar problem on the same side of the motorhome.
I told my husband about it who got all excited and said he knew what it was and promptly fixed it for me...
Mystified... I had to ask him...
He said it was a dash vent that was partially closed and just opened it up fully for me..OH MAN !!! He said they can also get stuff caught in there that will whistle... go figure, such a simple solution for my noise (this time...).
check it out ... hopefully it is your issue too :)