I am the original owner of a 2006 LS430 currently at 42K miles. Recently, there is light knocking (almost like a tap every 2-3 seconds) inside the transmission bell housing when in Drive (1st gear) or Reverse gears whenever the brake is applied or initially getting started on an incline. No knocking at all when Toyota WS ATF is below 130F temperature. No problems shifting and no knocking sounds after it gets rolling beyond 1st gear. I think the source of the knocking is in the torque converter as we have a long very steep driveway which we use reverse gear to back up on to enter into our home garage and we may have overstressed the torque converter. No check engine light. The engine is a V8 3UZFE 4.3L VVTI with a 6-speed automatic transmission. I have dropped the original ATF from the pan and refilled back with new WS ATF and double-checked for proper ATF level between 113F-130F. 

Is there something I can do to make the knocking stop by methods by myself at my home like changing a specific solenoid?

Thanks so much for your time, experience, and help.