Heavy, rhythmic clanking from drivetrain after rock encounter

[bkn1964]

I need help diagnosing this for my '99 540i 6 speed with 110,000 miles on it. I've been conducting internet and forum searches for days but haven't found anything yet.

I have a fairly heavy, rhythmic clanking (not quite clunking, but close) from the rear half of the car. It seems to mainly be under load (accelerating or decelerating). I haven't taken the car over 20 mph since the incident that caused it (below).

I was driving last Tuesday morning, still twilight, with fresh snow on the road. Going about 35 mph around a curve. Too late to avoid it, I saw an object in the road that looked like a chunk of snow/ice about 10" wide by about 5" tall. I drove over it with both right side tires. It was a rock and felt about like you would expect (almost like hitting a curb straight on at that speed).

I slowed down, turned down the radio, and listened, but didn't hear or feel anything obvious. I drove slowly (30 mph or less) the remaining 5-10 minutes to my destination, listening for anything abnormal.

As I got closer, easing up to a stop sign in first gear I could feel a pulsing in the shifter, rhythmic with the vehicle speed. The last few blocks it also started feeling like a tire was going flat. I pulled over and checked, but the tires were fully inflated. Pulling into the parking garage it started making an erratic, loud "pinging" sound from the drive train, like a wheel bearing was starting to bind.

I had the car brought home on a flatbed.

After checking for anything obvious, my initial diagnosis was a bad rear wheel bearing because it sounded like the bearing was grinding as I spun the lifted wheel. I replaced it (fortunately not too difficult of a job), but it wasn't the problem.

My next thought was the differential, but it doesn't "seem" to be a heavy enough sound (although it still could be). I also don't think it's anything slapping tin, as it's heavier than that and seems to happen mainly under a load. With the back of the car lifted, tranny in first gear, holding one rear wheel and spinning the other back and forth to each end of travel (about 4-6 inches), there's some slop and moderate clunking at each end of the travel (from exactly where I can't tell), but I don't know how much is normal. With the tranny in neutral and spinning rear tires opposite directions or both the same I don't hear any abnormal noise (grinding or anything else) from the diff or elsewhere.

I've been underneath and can't see anything obvious. Unfortunately I don't have my Bentley's manual yet, as I ordered it well before Christmas from Bentley's and after waiting more than a week for it I called to see the status and they told me they were out (thanks for the heads up, Bentley!) and to check back with them in mid-January.

Any thoughts?