How can I stop my Worcester 24i combi boiler vibrating?

Hi,
My boiler is almost 9 years old and now makes the most terrible, screaming vibrating noise when I switch the heat on.
It is not constant, it screams for approx ten seconds then ebbs away then starts again after a minute or so and it's driving me mad!
I have a basic homecare agreement with British Gas, but when the engineer came out he simply said "we don't fix sound" and suggested I use bluetack around the front door casing.
This doesn't work, even when I take the front panel off, it still screams. If I press the front of the boiler however, it stops.
I can't afford a new boiler, but can't bear the sound of it in a small house. Any suggestions gratefully received! :-)