humming sound

I own a 1984 123 300 TD, and when i accelerate up to about 20mph, then let off the peddle, I start hearing a vibration/humming noise. (sounds almost like if you were driving with all windows closed except one). thats how it started. now its progressively getting worst. now it humms and vibrates even when driving at higher speeds. Ive checked the tires, all tires are new, and ive checked all the bolts on the tires and they are tight PLEASE ADVISE