Hi Folks,

99 RX300 with 249k miles (been such a good vehicle)

Under medium/hard acceleration there is a knocking sound from what i think is the driveline/rear of the vehicle. Doesn't do it under light acceleration. I've checked transmission fluid levels and they are fine (read that could be torque converter).

Any thoughts or areas to look? I'm a bit confused on this one, although i'm leaning towards transmission.

 

 

If the knock still occurs, then you need to replace the gasket inside, as they can get rotted out.

 

Not sure if you have an AWD, but one other possibility is a loose U-joint on the axle denoted by the arrows. The exhaust I have is modified so yours will be a single exhaust tube going back to the muffler.

 

I had a similar noise, sounded like the front though. Single clack/tap every time I pulled out from a stop. Turned out the engine stabilizing strut went bad and the excess movement trashed the exhaust gasket. So be sure to check your engine mounts too.

 

This^^^^^

Knocks under acceleration are usually related to power train mounts going bad. As it's a metal box, sound tends to transfer in interesting ways.
Get helper, pop hood open, stand in front of running engine, in P, of course, and ask helper to rev engine. If engine clearly moves back/for, mounts are bye bye.
Otherwise, you have stabilizing bars in the rear, their bushings go bad and makes noise. That's long, LONG pry bar job to determine.

 

Or suspension components (shock tube rattling/knocking, etc)