How to reduce the whistle? 

step 1.You can take out the battery first, and then use the fine tuning 
bar to set the sound to minimum. 
step 2.Relocated the battery and turn on the machine. 
step 3.Slightly adjust the volume to a suitable size. 
you can buy a bottle of EAR LUBRICANT,It can make your ear and hearing aid 
completely closed.
 
Product Information
Brand Name Cofoe
Product Name Cofoe Inner ear hearing aids 
Model No. D-C-C1L
Color Orange,White
Material Plastic
Host Size 1.8cmx1.2cmx0.9cm
Net Weight 0.3kg
Gross weight 0.5kg
Category Customized
Battery A10
Calibration Automatic
Frequency range 200Hz~7645Hz
Working current 1.36mA
Compression rising time 48ms