It was more like the hum of a very happy bee that had found a rich well of honey in some globular flower, than anything else I can think of at this moment.

Where could it come from?
She laid her ear first to one of the doors to hearken if it was there--then to another.

When she laid her ear against the third door, there could be no doubt where it came from: it must be from something in that room.

What could it be?
She was rather afraid, but her curiosity was stronger than her fear, and she opened the door very gently and peeped in.

What do you think she saw?
A very old lady who sat spinning.
Perhaps you will wonder how the princess could tell that the old lady was an old lady, when I inform you that not only was she beautiful, but her skin was smooth and white.