This morning on the way home from work I noticed a whirring, almost electrical sound coming from the car. Paused the cd player to make sure it wasn't the stereo, and it doesn't follow engine speed like a bad ground would. I peeked under the car and under the hood before I left for work this evening because I thought I could hear/smell an exhaust leak. The only smoke I saw was from oil that had dripped on the exhaust pipe right before the p-side cat, but it sounds like that noise is coming from the cat as well. Any idea on what this sound is?