30,000 Dyna glo dual fuel vent free heater making a whistling noise?????

Ohiobuck

Please help guys. Have a Dyna glo wall floor heater installed today.connected to natural gas and Has a external regulater at 6" wc and connected thru 1/2 flex hose. Heater works great on high but the minute I put it below 2 or below on the dial it starts making a whistling noise. Heater is brand new so I wouldn't think the air inlet for orifice would be dirty. Any help guys? Thank you much for any info.