Central Heating noisy (But not the usual way)

Hi
I have a strange issue with my central heating that I cannot figure out nor can the last 3 Central Heating engineers I've tried.
My system is quite basic. Firstly it's microbore, it has a condensing boiler feeding a pump in the airing cupboard upstairs that splits into 2 feed and returns which in turn split into manifolds with 4 rads at the back of the house and 5 at the front. the system also has a header tank and a vent pipe into the water tank in the loft but the problem is this.
When it's starts up it will run for about 15 minutes with no issue and quite quietly. After about 15 minutes the noise starts, it sounds like air running through the pump, the pump becomes very noisy to the point we can't hear the TV in the lounge. It does this everytime we use it either hot water, central heating or both.
We cannot find any where air is getting in, we cannot find any air to bleed out of rads or otherwise, we cannot find any cause of this at all but it is a real problem as when the heating comes on in the morning it will eventually wake us up. Also it is worth noting it will then start cycling this noise with about a 2 minute break in between

Any help will be greatly apreciated as I said before severl heating engineers have tried and failed

Gahmusic said:

.
We cannot find any where air is getting in,

Click to expand...

have you checked air is not being drawn in the f/e vent pipe ?

I have to admit I have no proof that this is not hapening. It has been mentioned to me that this is a strange setup to have both a header tank specifically for the system in the loft plus a expansion vent into the main cold water tank.
Do you have any idea how I could check to see if this is happening?
I have checked the feed from the header tank and can feel warmth about a couple of foot up but past this the pipe is cool but unfortunately I cannot get to the vent pipe in a similar way.


Thanks, Gary

Gahmusic said:

It has been mentioned to me that this is a strange setup to have both a header tank specifically for the system in the loft plus a expansion vent into the main cold water tank.

Click to expand...

strange ?
should have 2 tanks one small one larger.
both have vent pipes from from the heating into the small tank.
one from the hw cylinder into the larger tank.

Do you have any idea how I could check to see if this is happening?

Click to expand...

hold the vent pipe in a glass of water and see if it draws the water into the pipe.


whats the condition of the water and the bottom of the f/e tank ?
what speeds the pump on ?

Hi
Pump is on speed 2 out of 3 the other I will have to check later at home.

thanks, Gary