They could hear footsteps, Filch running as fast as he could towards Peeves' shouts.
"Oh, move over," Hermione snarled. She grabbed Harry's wand, tapped the lock and whispered, "Alohomora!"
The lock clicked and the door swung open -- they piled through it, shut it quickly and pressed their ears against it, listening.
"Which way did they go Peeves?" Filch was saying. "Quick, tell me."
"Say 'please.'"
"Don't mess with me, Peeves, now where did they go?"
"Shan't say nothing if you don't say please," said Peeves in his annoying singsong voice.
Then, as you know, they turn around to face Fluffy who is glaring at them and about to attack. They were inside that room for quite a while listening to Peeves and Filch before they turned around to see Fluffy. Why didn't Fluffy attack as soon as they entered the room? Possibly because of Peeves' singsong voice, which might have started putting Fluffy to sleep. Fluffy didn't start baring his teeth until after Filch and Peeves had left.
Oh wow. Good catch. ;o Did he do it on purpose or was it just lucky? _________________
"This is a new garden over old flowers, and old makes old jokes take on the feel of the lore."