I thought I heard a noise, but maybe it was just my imagination.

imagination 是名词，由动词 imagine（想象）加上名词后缀 -tion 构成。imagine 来自拉丁语 imago（图像），所以 imagination 的字面意思是"形成图像的能力"，引申为"想象力"或"想象出来的事物"。