So I've got the block and head in the trunk, crank's on the floor behind me. I'm heading down into NJ to a machine shop referred by friend to get block tanked and honed, head redone. After I get up to speed on the NJ turnpike, around 55-60, I begin to hear a noise/vibration from the rear of the car which I'll describe as what I would imagine knobby tires would sound like. A low rapid drumming, thudding sound corresponding to to ground speed. This occurs the most under load, and if I back of the pedal, it ceases. If I increase speed and drop into OD, it lessens, until the load is reestablished. On the flat under less load at steady speed it becomes minimal. The noise seemed to increase somewhat over the next 20 miles. Stopped and checked tires and reached under to try and check play in drive shaft. All seemed well enough. Continued. The vibration grew strong enough to vibrate a piece of loose metal resting against the window on the rear window shelf.

3 weeks ago I rotated the tires front to back. Prior to that, I was beginning to hear (and trying to deny hearing) the beginnings of this noise when I'd take off from a stop or turning a corner under load. (a rapid rruh,rruh,rruh,rruh,ruh,rruhing sound. How do you transcribe noises into words? Let's make an audio dictionary to help identify these things.)

So I'm thinking driveline; center support bushing; rear end. - low oil or bearings.(ouch). U joints and center support bushing and bearing have less than 15K. Maybe rear axle bushings or other worn parts(?) allow differential input to rotate down under load, thus making undue load on rear U joint/differential connection, translating into this noise. Any ideas/prior experience with thie symptom? Pouring rain today so I'll drag the floor jack out onto the street tomorrow and crawl under.