The power supplies in these older systems inherently create some amount of acoustic noise sometimes. If you still have the stock fan in the Quadra and the noise is audible over it, that could indicate a problem with the power supply (but if you've upgraded the fan to a quiet modern one, the amount of chirping/whining heard is probably normal).
That said, first thing I'd do is go over and reflow the solder joints on all of the heavier/frequently stressed components (diodes/transistors mounted to heatsinks, transformers, connectors, the pins on the daughtercards visible in your first photo). You may also want to recap it: ISTR other members experiencing a lot of grief when recapping their Q900/950 PSUs a few years back. If you decide to go down that route, I'd strongly suggest that you pay attention to the ESR spec of the capacitors when ordering replacement parts, to perhaps save some headaches.
All of the capacitors at the outputs of the power supply are pretty much guaranteed to be low-ESR caps. If you're unsure about any of the other caps, each one has a 1-3 letter code on it that you can Google to find the datasheet (e.g. SM, SMS, SME caps are all standard electrolytic capacitors. One labelled LXF, on the other hand, would be a low-ESR cap, and needs to be replaced with the same).
