Boiler problem?
 

Hello

I've got a central heating system like this one
http://www.gasman.fsbusiness.co.uk/images/Yppic.jpg
The only slight difference is that the cold feed and expansion pipes were
combined into a single pipe about 10 years ago due to a blockage in the cold
feed. This was done by a BG man. The boiler is about 27 years old. Both hot
water and central heating work fine.

The issue is, that on start-up (twice a day; morning and afternoon), the
water is heated in the boiler and gurgles (loudly) into the expansion tank.
After this initial burst of activity, it operates normally.

Question is, why?

The thermostat on the tank seems to work ok. The pump seems ok. And as both
hot water and central heating work, I guess the 3-port valve is ok too.

Thanks for any comments.