I have a feeling this has already been posted, but I need to ask.

Just a few minutes ago, I was having some fun on a back, country road. I noticed that at one point when the car downshifted into 3rd, I heard a quick shudder of the car. I kept trying to duplicate this, and finally did pulling a hill. It was in 3rd gear at around 3,000 RPM (not exact, but close enough). It's a buzz throughout the whole cabin that you can feel. What is this? Can anyone help out?