I have an '87 420SEL which had a barely noticable noise while driving last week which has turned into a very noticable noise now. It started out at certain speeds on certain types of pavement. Now it will do it loudly at most any speed on all pavement. It sounds like someone blowing into a bottle neck- kind of reverberating through the car. I thought at first that it was a speaker problem, maybe some feedback from the amplifier. Then I thought it was a tire problem, but I have looked at all the tires for signs of uneven wear, blisters, etc. and can't find any. The only thing that will make the noise stop (momentarily) is to tap the brakes. Have I got a caliper or rotor problem?