Any ideas what it could be & also if I can buy a power supply that will be much quieter.
Its best for you to post your complete specs of your PC so we can suggest a PSU in the case thats problem, but i think you should start stopping components and identify whats causing the noise. Open your case to have less risk of something overheating, if the noise is present at idle, do not load the pc, just let it sit and start stopping fans for brief periods, the best would be to stay minimal like only CPU fan and the PSU fan and stop either briefly and see if you can pin point the noise, it could even be bad caps or coil noise, sometimes is hard to point the noise, but with a little patience you might be able to. My bet is on the PSU, specially with dual small fans, but you never know.
If its the PSU, check SPCR list of recommended PSUs, brands that you should look for are Seasonic X series (although some have reported buzz on them, i haven't had a single issues with mine), Kingwin Striker and Platinum, Enermax 87+, Corsair AX series.