I have a 172 MK1 w/ 28,000 miles on it. Recently I have developed a "whirling" or "humming" sound at 75mph. Its not the engine or exhaust because the noise continues when I put it in nuetral. And it isnt likely to be a tyre. Its particularly loud at that speed but I suspect that the noise is constant. Maybe its the rear axle, bearing, or a brake. I was curious if anyone else had this problem or some insight?