While the SUB makes some noise when powered up, it could be heavier than usual. There are a few things you can check:
1: Check the bottom vents of the machine. It could be there is dust or dirt packed in the fans at the bottom, causing the machine to make more noise in trying to cool the SUB Keg. You can clean the vents with a vacuum cleaner by hoovering out the vents from the outside.
2: Check if the issue repeats itself with a different SUB Keg. It could be caused by a faulty Keg not fully connecting to the machine, causing the pressurising noise to repeat itself consistently.
If neither of these options sorts the problem out, please get in touch and we'll have a look for you!