How many miles are on your car and when was the last time you replaced your spark plugs and wires? I had a very similiar sound happen to me not long ago but only when I WOT'd it. It turned out to be my spark plugs and wires. I tested the wires with an ohmmeter and found one of the plug wires to have very high resistance. I had 80,000 miles at the time and had never replaced the plugs or wires before. The plugs were all carboned up so I know that factored in as well.