The easiest one to find though is if you've cranked a fine tuner down so that the little metal bar under the screw is actually touching the fiddle top--that's why you should back off all your fine tuners now and then because if you have a slipping peg or string, you will eventually get there trying to compensate with the fine tuner.

Have you tried the little rubber donut thingies where the E string touches the bridge? Reduces harshness.

Finally, how are your strings wrapped in the peghead--are there loose ends rattling around there?