The Howling-1987 300TD

[raduoos]

I have a mystery howling noise in my 1987 Mecedes 300TD. When it started, I thought the muffler was going out, but I liked the low rumble. Then it got increasingly louder. Today, it is loud at 15mph and at 60mph it is unbearable. Because I had already replaced all four wheel bearings, I first suspected the rear differential. When I dropped the gear lube, it came out nice and clean. I refilled the rear transaxle with transmission fluid to see if the noise would change. After 100 miles, I replaced it with clean gear lube. There was no sound difference and the fluid was clean. Then I suspected the center bearing of the driveshaft. I had the entire driveshaft assembly replaced with no difference. It sounds like it is coming from the left rear, but my ears cannot be trusted. I replaced the left rear half-shaft and the sound still did not change. "Howling" is the best description I can think of. The sound does not change when I accelerate or decelerate or swerve. It gets progressively louder with speed, but not with engine or transmission RPM. Does anyone have any ideas?