这个俺可以发言。租户说

二楼水龙头可的时候，楼下棚顶有水滴声。

the fact is the plastic pipe expends and knocks the wood below when hot water running through especially when it is cold outside.  Turn hot water on and run for a while, the "dripping sound" will get irregular and dispear.  of course, need to check celling is still dry.