Baxi Combi 130HE boiler problem

Hi all,
We have a Baxi Combi 130HE boiler which is roughly 18 months old.
Recently we've noticed it occasionally making a very disconcerting noise and we're wondering if it's anything to be worried about.
I can only describe the noise as a short-lived low loud vibrating rumble as if a large motorbike were revving in the driveway. If you're downstairs (which is where the boiler is located) you can feel the rumble through the floor. It doesn't last long (a second or two), and there's no evidence of a fault of any kind on the boiler's display.
Any ideas anyone?
Thanks in advance for any help.
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