Harman Kardon subwoofer whistling problem (HKTS 210SUB - HKTS 16)

I bought the HKTS 16 5.1 surround set from Harman Kardon 3 days ago. I noticed that the sub sometimes produced this constant noise (around 400 Hz) after I played some music. Is there a way to fix this or is the sub already broken ? SOLUTION : I kinda solved it. It happens every time I use my sub, I turn it off and the back on. To to not have this problem after I turn the sub off, I need to wait maybe 10 minutes before turning it back on. That means I can not use the "Power On Mode" feature.