Anyone know what may cause this? My car is still under powertrain warranty and I'm wondering if I should bring it in for this.

A loud, low thud or thump sound comes out from what sounds like my rear differential when shifting into Drive.

 

I wouldn't hesitate to take it in to Lexus...best not to take chances, especially if still under warranty!

I do know of issues with the throttle sensor causing jerking motions when shifting from park to drive or park to reverse, but I would definitely take it in if I was you.

 

How good/bad is Lexus service departments about voiding warranty based on maintenance?

I have service records on my car from previous owner, but he did the 60k service at a private shop. Do they have any basis to void my powertrain warranty because of that?

I'm guessing they may look at the last time ATF and Differential Fluid was changed.

 

If it's serious (meaning AT or diff rebuild/replace) and you give them an out, they will probably take it. But that would have to assume the worst. Any modern automatic, even the US makes, shouldn't be sweating hard to get to 60 or 70K miles (and I hope a lot more). When you hear hoofbeats, think horses first, get the fluid changed and make sure the levels are up to proper range - might just make the sound a lot better or disappear altogether. Might still be something like a U joint which might not be covered under the warranty but is a lot expensive to fix than an AT.

All the best, good luck.

 

Sounds like a U joint. Not a big deal...but certainly have them check it out to be sure..