'99 Corolla Making a strange noise

I drive a '99 toyota corolla that has 78,000 miles. When I take my foot of the gas after accelerating at higher speeds, I hear some sort of humming noice that I have never heard before. It almost sounds like the car is having a difficult time coasting. I brought it into a local gas station and the mechanic said he didnt hear anything. Other have been in my car and have commented on the noise. What could the problem be?