Difficult to keep within a £20 budget so just thoughts about things that have no cost..........
Is the case vibrating in resonance with one of the moving internals? - Cure - tighten any loose screws, re-route and tie chunks of cable. Deaden a tinny surface (even a house brick on top!).........Be careful not to impede ventilation.
The fans themselves, (any or all) may be dirty. -Cure switch off and remove each in turn. carefully sweep clean with a soft artist type paintbrush. Pay attention to any heat sink under a fan, the vanes can become stuffed with dust.
Noisy HD..... It should not being making a noise, but you could make sure it is properly screwed into its cage and not just dangling from one screw.