I have an 87 740 with 200000 miles. At high speed, around 80mph, a strange noise starts, like a combination of growl/howl with frequency modulation and vibration from underneath the car. It diminishes if I let off the gas, but does not disappear, returning as soon as I apply even slight throttle. If I use the brakes to stop the car it also adds something like a worn rear rotor noise. The noise doesn't seem to go away until I stop driving for a while and the engine is cold again. It is hard to describe the noise, reminding me of when you have a big hole in your muffler. I checked, I may have a slight exhaust leak around the exhaust heat shield but nothing major. What it is most strange is I did not have it until I recently started taking care of my car: changed the catalytic converter, timing belt, front brake pads and calipers, balanced and rotated tires. It started just after all this, never have it before. Could someone help me if experienced the same problem?
Thanks,
Lis