Burnt MOSFET and buzzing power supply

This issue hit entirely out of the blue. I was sitting at my desk, my printer on the table behind me when I smell smoke, coming from the printer. Panicked, I unplug everything and the smoke stops. upon close inspection of both the mega and the ramps, is seems that the only damage is a toasty-looking D8 mosfet, which was leaning against the D block and melted a small patch of the outside. it didn't seem to have damaged anything else though, so I tried connecting it to the computer. It came on okay, all temps and responses working, so I tried plugging power back in, only to discover a horrid electrical buzzing noise coming from my power supply. I unwired it from the printer, and it seems that the buzz starts when plugged in, then continues for a few seconds after being unplugged before petering out. Nothing on it looks burnt and nothing is noticeably moving. What happened here?! I don't want to do anything else with that power supply before i get answers.

Try removing both wires from the green connector, switch PS on and see if the noise goes away - just to test if the connector has a heat short in it.

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Waitaki 3D Printer

No joy, the power supply makes that noise when it's plugged into wall power, even when it's not wired to the printer at all. This is really a weird malfunction, as the printer wasn't even running when it happened.

Upon opening the power supply and careful listening, it seems the buzzing noise is coming from the large transformer in the middle.

I replaced the power supply, and checked a couple connections. A heated bed is supposed to have about 1.2 ohms resistance value, right? That's what is came out as, no shorts, so that's alright. However, when I ran an ABS print on the new bed, I started to smell burning. I pulled the plug before anything could burn out, but the D8 mosfet was starting to look a bit brown around the edges. So it's not the power supply, and it's not the mosfet, and it's not the bed. Could the Mega be causing this problem?

have you got a fan permanently blowing over the Ramps (especially D8 and the yellow polyfuses?) AND, a heatsink on D8 mosfet?

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Waitaki 3D Printer

If the mosfet was touching the D8 block then you probably have some melted plastic it that burns. Some boards have MOSFETs with borderline ratings.
If that seems to be the case then I'd suggest replacing it with something like [www.reprapsource.com]