There were still a few surprises, as a squeal here and there in the dark announced...

announced在这里不是过去分词，而是过去式，为as分句的谓语动词。

as引导比较分句，意为“正如...那样”，修饰前面整个主句的内容。announced在此为比喻性用法，意为“表明”，如下句中的用法：

The sound of the door slamming announced his departure. “砰”的关门声表明他离开了。

There were still a few surprises, as a squeal here and there in the dark announced...

正如黑暗中不时地传来的尖叫声表明的那样，仍然有一些意外的事情发生。