Shir Hashirim is Kodesh Kodoshim.
http://hebrewbooks.org/pdfpager.aspx?req=37951&st=&pgnum=427
Also, the fact that Shir Hashirim uses the relationship between a man and a woman as a moshol for the relationship between klal Yisroel and the Eibishter makes it particularly dangerous to make songs out of it (although most learn that the issur applies to all of Tanach).
IIRC, there’s also a teshuva from R’ Moshe about it; I’ll try to find it.