High Frequency Problem

Hi everybody , i just moved to studio where the owner was the victim of acoustician :(, In the take room (17' x 22' x 8') all parallel walls but its 45 degree from the ceiling to the walls ,While i am recording i have a harsh high frequency (6k to 11k )and specially if i am recording a Choir it is more noticeable . And it is annoying . like you have sssssss and zzzzzz on the whole take . The way the studio been insulated (Sadly) its existing concrete wall ,then 2 x 1/2" drywall , then 2" owen 703 insulation ,gap 2" and then the burlap . The Whole studio done on this way , Except the floor (Hardwood laminated). Please i need an advice how to eliminate the high frequency from the take room. Thank you in advance . Alex