noisy pump driving me mad!

I hate computers. Just asked a question, with a long explanation, and then managed to delete it all. Not having a very good morning!
There is a pump in our loft making a very rhythmic, pulsating noise that is pretty noisy. Noisy enough to be stressing me out!
We moved here last november and it did do it over the winter periods quite a few times but always seemed to turn itself off after a while. It then seemed to stop altogether being a problem when we had the central heating turned off. The hot water then packed up so we relied on the immersion for hot water. Now the central heating is back on it seems the pump problem has come back. We are due to have a new boiler v soon as current one is very old and according to boiler man, now not legal as in middle of kitchen. Not sure if the problem is the pump or boiler.
My husband went to work this morning and ran shower for a few minutes. The pump then started and has now been pulsating for about 2 hours. Yesterday when it did that it stopped when we ran the hot water tap in the bathroom, but that trick doesn't seem to be working.
Why is it always on weekends?
Is there anything I can do to stop the pump? Is it safe to leave the house with it constantly going?
If anyone can help I'd really appreciate it!
Thanks, Kate

Is the pump for the shower ? if so there should be a switch to turn the pump off, where the switch is depends on whoever installed it, if your lucky it will easy to find if not you will have to go into the loft and have a search, dont leave it running , the pump will burn out.

thanks for replying so quickly. I have just had my shower and the pump has stopped. HOORAY!!!!
The pump is for the shower I believe. Is there anything obvious that it's likely to be? To be honest when we got rid of an en suite that the last owners installed I'd not be surprised if the pump has been fitted wrongly or something similar. The work behind the smart tiling and fittings was really awful. Drainage pipe from shower running away from shower at 45 degree angle...upwards? Even I know that's not going to let the water drain away!
Thank you again

Probably the flow switches sticking or trapped air in the pump, if the plumbing is that bad I would surprised if the pump is installed properly, find the isolation switch for it, running it with no flow will burn out the motor and you may have an expensive one, at least you will be able to turn it off when it sticks next time.

thanks I'll be calling the plumber tomorrow. Thanks again