He's left a message saying that he has someing important to

楼上的胡扯!
saying 在这里引导了一个同位语从句,解释说明massage的内容.
这种结构很常见,多用词read\say等.
如：
there is a startle news today saying 50 men killed in a big fire.
分词做状语,状语是副词性的,与原句是完全不一样的.
分词做状语有一个很大的特点是分词的逻辑主语与主句的主语必须一致.
上句中主句的主语是he,但分词的逻辑主语是 massage.很明显不一样.
如：
hearing the news,they set off for shanghai at once.(状语)
being a layman in matters of culture,i would like to study them.（状语）
另例：
this is my pleasure,doing you a favour(解释pleasure).
he enjoys the exercise,swimming in winter(解释exercise)