Can someone give me a bit of a tutorial on checking the flex discs outside of just looking at them for cracks? I'm getting a rumbling sound and a vibration/thumping feeling at the floor by the transmission shifter when the car is in second or third gear and you step on it like you want it to downshift or if you are going uphill. Funny thing is it only happens between about 25 and 35MPH. Any faster or slower and no issue. Also when it starts vibrating, you back off the accelerator and it goes right away. There is no vibration, noise, or anything at highway speeds (smooth as silk)

I'm thinking flex discs, driveshaft bearing, or maybe the U-joints (they do clunk).

BTW, car is a 1983 300D with 278K on it.

Thanks,

Glenn

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2012 Mercedes ML350 Bluetec 102K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 238K (mine)
1998 Volvo S70 T5 Turbo 211K (kids)
1994 Ford F150 4WD 246K (firewood hauler)
1983 Mercedes 300D 384K (diesel commuter)