Why did Scrooge draw back his bed curtins?

He drew back the curtains himself because he didn't want the next spectre to surprise him (previous visitors had drawn back the curtains). In other words, he was on the look out and waiting!

Textual Evidence

"But, finding that he turned uncomfortably cold when he began to wonder which of his curtains this new spectre would draw back, he put them every one aside with his own hands, and lying down again, established a sharp look-out all round the bed. For, he wished to challenge the Spirit on the moment of its appearance, and did not wish to be taken by surprise, and made nervous."

I was told he heard a sound is that correct?

Yes Mickayla.

He heard a sound and wanted to see what it was. So he does and then he faints cause he saw a ghost.