Thumping sound under car while turning- vague I know - Help!

[apache31]

2003 S80 T6
30,000 miles

-thumping sound while turning
-sounds like it is coming from underneath the car
-can feel it in the gas pedal
-can feel it in the steering

It is intermittent - goes away for a few weeks and then comes back louder.

This is my mother's car. She asked me to ask you guys because she is aware of the success I have with you on my own car. This is probably the most vague description I have ever received about a car problem and apologize. Taking a chance that perhaps someone has experienced the same problem.

Thanks in advance,

Rich