Ideal Combi Response 80 noisy problem.

My Ideal Combi boiler is making an odd noise. It starts about 10 minutes after switching on the heating from cold and lasts for about 1 to 2 hours, then it shuts up. The noise sounds just like the noise you get when blowing over the top of an empty beer bottle. If I listen carefully it sounds maybe likes it's bubbling a little. Normally I wouldn't worry but I'm fed up with being woken up in the morning by it. Occasionally it makes a loud humming noise which stops after about 30 seconds to a minute.
The beer bottle noise persists when the heating section shuts down and only goes away a couple of minutes later when the pump shuts off. According to the manual, the pump is the only thing that is still running after the gas shuts off.
My radiators have got very hot recently as well. I've had to turn the thermostat down which is completely unheard of in my house. I have 5 radiators with thermostatic valves upstairs and three rads downstairs with normal valves. The double rad in the lounge takes a while to warm up with the back section getting hotter faster than the front. All other rads get hot pretty quickly.
I have British Gas heating care and I've got a man coming out to look at it. He's going to replace a thermistor and the pump but I've no idea if that is the best course of action. Anyone here have any better ideas?
Oh, the boiler is 5 years old and was serviced in January.

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