Why does my Nikon D5100 keep making weird noises?

It's only just started doing it… After taking a picture on 'Bulb' shutter speed. It sounds a bit like a really old computer. I think it might be the camera loading but i dunno?

It's either doing a sensor clean (sounds a bit like a whirring noise) - check to see if you've inadvertently turned on 'sensor clean on shut down'.

It's possible that if you have long exposure noise reduction (LENR) function switched on, it is doing a second exposure after taking a shot in 'bulb' mode - though LENR isn't usually noisy.