Click!
 

I have fixed umpty-ump issues in my 2003 TL, just have one left.

If I accelerate briskly I hear a "click" from the passenger's front wheel area.

I thought the upper control arms were a bit loose, so I replaced those, no improvement.

All the other rubber parts look real solid. I did replace the passenger-side engine mount, it was failing, and that tightened up the throttle response, but the click is still there.

Any idea what could be clicking?

Not quite the same symptoms as you describe, but my front brake pads regularly rattle when going over bumps.

odd to have a side motor mount fail on its own, normal is front goes-- then after some months of driving that way- the side mount fails

Clicking can be in the CV joint bearings
Have you listened with a mechanics stethoscope yet? Easy way to find noises!

^

CV Joint failure is typically described as a 'click'.