Baxi Combi - Hot Water Whining

My partner and I recently bought a 2-bedroom maisonette. The previous owner had the central heating replaced in August 2002, with a Baxi Combi 80Eco.
When we moved in, both the water and the gas had been turned off at their stopcocks, and had been that way for maybe a week or more. I turned the gas and water back on, read the manual for the boiler and when I had satisfied myself that it was safe to turn on and test it, I did so. However, upon testing the hot water, we were met with a loud, high pitched whine which kicks in a few seconds after the boiler lights. The noise seems to be coming from somewhere near the boiler.
The boiler appears to be doing its job - temperature of the rads and water is fine. The pressure is within the operating limits, but maybe a little high. I can't figure out how to release the pressure though. Only problem I found was that the rad in the bathroom was virtually full of air. After bleeding all of the rads, the noise was still there.
The central heating is virtually silent, apart from a bit of creaking, which I have isolated to one of the rads - If I close it, the creaking stops and the heating runs silent. Hot water is still the same though.
The whine is there regardless of whether the heating is on or not, but it does seem to be slightly quieter when the heating is on, and it takes longer to start whining when the heating is on.
In addition (bear with me, nearly finished!) when the heating is on and the hot water is used, there is a creaking noise from the boiler when the tap is closed, a bit like pressure building back up and the pipes are giving a little. This noise only happens when the heating is on, and doesn't seem to be related to the creaky rad mentioned above, since it happens when it's closed.
If you read all of that then thanks for listening!
Can anyone offer any advice before I call someone in to take a look? Thanks in advance!
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