Central Heating pump knocking?


A couple of weeks ago I refitted a radiator in our bathroom after it had been tiled. Coincidentally, during the back end of last week I noticed a knocking sound at the same time as I could hear water being pumped through the radiators in the house. I wasn't concerned as I thought maybe some air had got into the system (it is a sealed system with a 8 year old Saunier Duval Thelia Twin combi boiler) and I would go around the house and bleed all the radiators. Last night, the knocking became continuous and even louder and I now realise it isn't coming from the pipework or the radiators it is coming from the boiler but is being amplified through the piping so that it can be heard throughout the house. The noise appears to be coming from what I think is the pump which seems to be an integral part of the boiler.
The noise is a regular loud knocking sound and stops when I switch of the boiler, but starts when either the hot water only or heating and hot water settings are selected. The radiators are still warming up and the water is still hot so everything appears to be working okay apart from the loud noise which is so disruptive that we had to turn the boiler off last night in order for us and our children to get any sleep.
Our heating system is insured but an engineer won't be out to look at it until Thursday, and then I suspect we will have to wait another week while they get whatever spare part is required.
If this is a mechanical problem with the pump is it likely to be dangerous to leave it on? Is the problem likely to be the bearings in the pump or is it possible that trapped air could cause this noise? I ask this as if it is a problem with the pump I'm surprised that the radiators all around the house are still hot i.e. the pump is still working.
I would appreciate any suggestions on what the problem might be and anything I could do to fix it or minimise the noise before the engineer arrives later this week.
Thanks.
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If you put the tip of a screwdriver on the pump casing and put your ear against the screwdriver handle, that should confirm that the noise is coming from the pump. It could be worn bearings and isn't dangerous. If it is possible to change the speed of the pump you could try running it at a slower of faster setting. I doubt that air would cause loud knocking. An old blanket wrapped around the pump might muffle the sound if it is safe to do that.
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snipped-for-privacy@hotmail.co.uk wrote:

Could be water too hot and creating steam in the pump. Have you turned the boiler Temp up lately? Have had this problem on my oversized boiler.
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As far as I'am aware the temperature hasn't been turned up. I have also tried turning down the hot water and heating temperatures but this makes no difference.
When I arrived home tonight (to the constant, regular, loud knocking noise coming from the boiler in the garage) I noticed that all the downstairs radiators and pipes were cold and I immediately assumed that the heating was no longer working. However, upstairs all radiators were hot! How can this be?
Is it possible that the knocking is being caused by a "switch" that is allowing hot water to the upstairs radiators, but not downstairs?
I'm even more confused now.
Paul.
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snipped-for-privacy@hotmail.co.uk wrote:

are heating is due to thermal syphoning.
--
Malc

"AFB Mr Tracey."
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