the purr of an engine.

• 机器的隆隆声

the purr of an engine.的用法和样例：

例句

1. We heard the occasional rumble of a passing truck.
   我们偶尔听到卡车驶过的隆隆声。
2. When the train was out of earshot he got to his feet.
   当火车的隆隆声在耳际消失之后，他才站起身来。