You will need to trace your pipes from the pump to find out where it goes. While about it, give any jubilee type clips a tweek just in case the air is getting in at a joint. Also do a visual check to see if there is any sign of a leak, however small. If you have a calorifer in the circuit, then check to see what pressure it's set to.
If you only switch on one tap at a time, noise can be generated by trapped air at another tap - so you need to open all taps simultaneously and then, when the water is running clear and fast, switch each off in turn. This will purge your system of any air.
Then watch closely to see if the noise returns - and if so, start thinking about where it's getting into the system.
Good luck!