From Chapter 28, 'Snape's Worst Memory':
The very floor of the office shook. Umbridge slipped sideways, clutching her desk for
support, and looking shocked.
'What was — ?'
She was gazing towards the door. Harry took the opportunity to empty his almost-full cup of tea into the nearest vase of dried flowers. He could hear people running and screaming several floors below.
(The "BOOM!", for the record, was Fred and George setting off fireworks.) As for the fake bit - to begin with, it seems quite likely as Snape was a spy and also hated Umbridge just as much as anybody. Further, in Chapter 37, it says
“Harry, you know that Professor Snape had no choice but to pretend not to take you seriously in front of Dolores Umbridge,” said Dumbledore steadily, “but as I have explained, he informed the Order as soon as possible about what you had said. It was he who deduced where you had gone when you did not return from the forest. It was he too who gave Professor Umbridge fake Veritaserum when she was attempting to force you to tell of Sirius’s whereabouts...”
Another relevant passage is from Chapter 32, 'Out of the Fire':
'Ah, Professor Snape,' said Umbridge, smiling widely and standing up again. 'Yes, I would
like another bottle of Veritaserum, as quick as you can, please.'
'You took my last bottle to interrogate Potter,' he said, surveying her coolly through his
greasy curtains of black hair. 'Surely you did not use it all? I told you that three drops would
'You can make some more, can't you?' she said, her voice becoming more sweetly girlish
as it always did when she was furious.
'Certainly,' said Snape, his lip curling. 'It takes a full moon-cycle to mature, so I should have it ready for you in around a month.'