Plumbing question? Worrying noises from my water tank

Perhaps there might be a plumber on here who could shed some light on this issue?

I'm not sure if this is serious or not, but it seems to have only developed in the last 6 months or so. Perhaps since the really bad freeze? Can't be sure.

Basically its this:

I set my heating to come on before I get up for work. Its usually on for 30mins before I get up.

After about 25mins there is an almighty whooshing sound above your head in the attic. Sounds like a lot of water rushing somewhere. At first I thought it was really heavy rain, but it happens every morning now. It can also happen during the day as well. Usually last for 20 or 30secs.

You can even here it in the living room. Sounds like its coming from behind the fireplace brace too. I don't have a back boiler by the way.

Any ideas? Should I get it checked out?

No-one any idea at all?
Looks like I'll have to call a plumber and try to replicate it when he's in the house.

Is it an old house ? in our house when the toilet is flushed its like living in a water park the racket that goes on for about 5 mins , i have the back bedroom so i can hear the water rushing everywhere and what sounds like the tank filling or something , although we have two tanks .

No problems though so i assume its just an old system .

It was built in 2000.

Please do not bump posts.