To stop feedback, we need to stop the leakage of sound. Check the earmould is seated properly in the ear by putting your finger on the earmould and gently wiggling it into the the ear a little more. This may stop the whistling - at least temporarily.
If it continues, contact your hearing centre for an impression appointment so you can get a new earmould.
In the meantime, use a lubricant such as Auragel to 'fill in' the spaces between the mould and the ear. You can get Auragel from your Hearing Australia centre or some chemists. Alternatively, a light smear of Vaseline or K-Y Jelly on the mould can help, but check with your doctor first before using gels if there is a history of ear infections or skin problems.