Many cat harnesses have velcro straps. The sound of velcro can be startling to some cats, but you can make positive association with the sounds of the harness.
Here are few steps how you can do this:
1. Start by simply fasten and unfasten the harness straps then give your cat a treat (or using another action that motivates your cat).
2. If you notice your cat getting scared, increase the distance between your cat and the startling sound (for example, go to another room) until the sound no longer scares your cat but is still audible. Fasten and unfasten the velcro at a distance and give your cat a treat.
3. Gradually decrease the distance to the sound. Repeat this step until your cat is comfortable with the various harness sounds.
Give your cat time to adjust 😻