Your tone at 17 Hz may be twice the low shaft speed of 8.19 Hz, depending on your frequency resolution. The low shaft speed is 3500/(60x7.12) = 8.19 Hz. You could possibly find side bands that modulate the gear mesh frequency (rotation rate x number of teeth). For instance 40 teeth on the...
You could use a fiberglass blanket with a 6 inch air gap behind it to obtain a high abosorption coefficient at low frequency. The fiberglass should be 3 to 6 pounds per cubic foot and be encased in a 1mil vapor barrier.
