Close encounters of the third radiator

There's a scene in Raiders Of The Lost Ark that I've always found particularly disturbing (and no it's not the melting faces - I used to be oddly amused by that as a kid). It's probably about three quarters of the way through the film, after the ark has been captured by the Nazis and is being transported to the secret island hideaway on a submarine. The ark is inside a wooden box, emblazoned with a swastika somewhere in a hold. As the camera draws into towards the box, the swastika begins to burn and several rats around it begin to look distinctly alarmed. And all the while there's a strange noise pulsing away in the background, underscoring the strangeness of the event, rising to a crescendo before the abruptly cutting to the next scene. It's a sound effect that seems especially beloved by science fiction and horror films, and short of a rising chorus of violins, it's a particularly effective way of raising tension in a scene.

The radiator in my bedroom makes near exactly the same noise.

It started a month or so back, probably after the central heating was turned on, waking me up in the middle of the night. It's a very peculiar way to be woken up - I had to look around to reassure myself that there were no strange flashing lights outside. Nothing untoward happened that evening though, and the noise subsided as suddenly as it started. Since then it's reoccurred at seemingly random intervals. I'm sure there's a perfectly natural explanation why this happens, involving turbulence, heated water, air bubbles, and a decided lack of a charred swastikas, but I think life would be more interesting if I discovered the ark of the covenant hidden under the floorboards. Obviously what I really need is to keep a rat handy so I can see how alarmed it appears when the noise starts again.

Hmm, there's a pet shop only a short way aways...