humming cold water feed pipe

would anyone with plumbing experience be able to help with the following problem.
we recently completed renovation of our house including installation of pressurised plumbing system. we are experiencing very loud humming noise from the cold water feed pipe in attic everytime the pump is activated (flushing toilet, turning on tap etc). pump is located on ground floor. this humming noise can be heard throughout the house. it disappears once pump stops. we also have the pump switching on intermittently day and night even when there is no call for water. pump has been replaced as part of the attempt to fix problem but to no avail. is there air entering the system somewhere. we have checked for leaks and running cisterns etc and these seem to be fine. thanks .

Is the noise you're hearing an amplification of the noise from the motor itself? How is the pump fixed to the pipes on the inflow and outflow? Are flexible connectors used?

If not that, it is likely water hammer

Thanks Leo. Fixed copper piping on inflow to and outflow from pump.
Definitely not flexible connectors.

The noise is almost certainly caused by a longish copper pipe vibrating freely to the pulses of the pump or the water pressure. It probably isn't banging against anything just acting as a conductor / amplifier. Find the offending pipe and damp it by fastening it to solid wall, rafter or joist as the case may be. You'll need a couple of clips like [broken link removed] to fix the problem.

google "water hammer arrestor"
Its a fairly common problem that is caused by water filling quickly in to you water tank and then suddenly getting stopped when the tank is full.

I have the same problem and have often meant to install an arrestor, but just haven't got around to it yet!

Switch to flexible connectors on the pump, that'll isolate the copper piping from the motor vibrations.