If you happen to use ventilation inside a mansion-type building with airtight spaces or if there are strong winds outside, it is possible for the air to flow through the drain hose, which may emit sounds upon starting the AC unit. 

The solution is to create a flow of air by trying the following steps:

1. Stop the ventilation fan

2. Open the air supply port

If the sound persists, there is a tool called “gyakushiben (逆止弁 - check valve)” which can be purchased at a local hardware store. This tool prevents outdoor air flow by attaching it to the drain hose, which usually resolves the sound problem.※Please note that if you open the air supply port upon starting the AC unit, it ensures smooth operation and also lowers the usage of electricity.