UPDATE: I haven't yet gotten a replacement PSU, as I wanted to clean out the PC of dust first, but now I'm in the market again.
Do I really need a 600W power supply? The biggest draw would be the Nvidia Geforce GTX 960, which I've been told needs a 400W minimum, but is there a way of testing how much power it uses at peak? Maybe 500W would be sufficient?
Also, what brand of PSU is LEAST prone to fan rattling? It seems that the Corsair rattled a lot ever since I first got it, like a fluttering noise, but despite cleaning out the PC, the noise from the PSU is even worse, like a sort of intermittent creaking or resonance.