1984.5 245ti, ive been having a wierd vibration lately, mostly in 4th gear, but it can apear in all gears. basically, go through all gears, get to 4th and if im not either accelerating a little or completely off the gas i get this deep hum, kinda loud and i can feel it through the whole car, also happens momentarily when i first get off the gas, anytime i let off the pedal in higher gears above about 20mph i get this, any ideas? i thought maybe it might be something to do with the driveshaft but i dont know, also, my tranny whirs a bit in 4th, then stops when put into od, could this be a tranny issue?