This is just a stab in the dark, but is there any way that the sound you hear is from the helicopter rotor?

All that I can suggest is to carefully re-read any ReadMe files or notes that came with the helicopter.  Could there be a switch on the panel that controls this sound?

I found an add-on some time ago that provided a rotor sound using a toggle switch on the panel.  It wasn't a part of the sound package.

Keep us posted.  If this doesn't help, someone else may come up with the right answer.

Regards,
JerryH