I had the same problem with connecting my Dell laptop to my RG20 Raven amplifier. As suggested above, I verified that it was a ground loop problem using a 3 to 2 prong adaptor.

I am going to purchase the Kensington Noise Reducing Car Audio Auxillary Cable on Amazon for $14. I think it is a better choice than the Radio Shack product recommended earlier post because it has 3.5 mm male plugs on both ends. The laptop and amp have complimentary 3.5 mm females sockets.

I might add that it is not a good idea to defeat the ground plug other than for troubleshooting purposes. The ground plug is there for our protection.