Our 1995 Mercury Cougar has developed an intermittent or sporadic high frequency/pitch vibration that when it occurs, simulates the sound of driving over road grooves or a washboard. When it first occurred, this is what I thought it was, just road grooves. But now it occurs sporadically, seems mostly under acceleration, and only for a short moment. There doesn't seem to be any predictability to it, but it does occur regularly over smooth road. Could this be a transmission problem? Thanks for any help.