I have a 1991 Jeep Wrangler that has about 115000 miles and quite a bit of wear and tear. This problem is that about five minutes after I have started to drive my jeep, it makes a whistling sound almost like a supercharger or a turbo. This noise only occurs when I am driving at around 35mph to 45mph. If I hold a steady speed of 40mph the noise starts very softly then builds up to a very loud noise. If I give it more gas or let off the gas the noise softens but does not disapear. After some time goes by it may stop but it soon returns. What is going on?