Last night my son reported knocking coming from driveshaft area. Brief test drive but am at work today. Occasionally at start up or around a curve a hard knock like someone is under the car with a hammer and hits a time or 2 or 3 on the floor. I am thinking u-joint or support bearing or transmission mount? Ideas? Thanks.
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'86 244 160k, 88 245 185k, 90 245 153k, 90 745 waiting for me to fix

More likely it's an exhaust pipe banging around. The tell-tale is what you wrote: "...Occasionally at start up or...." The engine shakes on start-up, and that rattles the exhaust pipes along their whole length. It's probably also loose enough to move around when subjected to G forces ("...or around a curve....").

If it was the U-joints, support bearings, or transmission mounts, it would be more likely heard when you accelerate strongly, or shift between D and R, not when starting -- when you start the car, the transmission is in neutral (or park, same thing), so there's no torque on the drivetrain. The sound under those circumstances you didn't mention, so I assume there's no sound then.

Check the exhaust pipes and especially their hangers/supports.

Of course, it's still possible to be a drivetrain issue, but start checking the more obvious, and easier/cheaper to fix things first (exhaust pipe) before you start disassembling U-joints, etc.

Good luck.

My 240 wagon has done something similar - only when fully loaded and accelerating out of a bend. I suspected the centre bearing, but it looked OK. When changing the exhaust I noticed the centre (expansion) box has a rubber 'bump stop' above it in the floor - so it is expected to touch the underbody. Also the rear of the centre box is quite close to a step in the underside of the body - so if it moves slightly it could hit.

Since changing the exhaust (due to an early failure - replaced under warranty) it hasn't happened again - but I don't know for sure it won't, as I never really found the cause.

I wonder if this could be due to worn motor mount. My son went to change the oil and the filter was on the mount. I got a new one a few days ago from FCPGroton. Maybe it IS the exhaust hitting due to the mount being bad.
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'86 244 160k, 88 245 185k, 90 245 153k, 90 745 waiting for me to fix

Most likely a frozen U-joint. Easy enough to DIY, but take heed: the driveshaft is 2 pieces, splined together in the middle. The assembly is balanced as a unit and ther are arrows scribed in each half to indicate their alignment. Put 'em back together even one spline tooth apart and you'll likely have bad vibration problems. DAMHIK.
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Thank goodness we don't get all the government we pay for. -- Wiley Post