The pump sound is always there and never changes. However, maybe you don’t hear it in some facilities with different layouts. I’ve used a few facilities and always heard it.
For the loud noises from the vibration, that happens while they’re actually doing the scans. Different portions of the scan create distinct sounds, though I don’t know which sound goes to which measurements. There’s gotta be a YouTube video about it.
The sounds have a harsh buzz to their tone, like somebody is playing sound effects through a Tesla coil.
I’ve told people that getting an MRI, particularly one where you need to go head-first into the tube, can feel like an alien abduction!