No ABS.  It continues and changes speed with the motor.  It is not a
shuddering, lugging sound like trying to start out in the wrong gear,
just most noticeable when in gear and rolling at or near idle RPM. It
is also not a rock in the tire or something like that - most definitely
a mechanical clunk.  The motor also always has some of what seems like
valve train noise, but that is there even when the car is warm.  I have
heard that an exhaust leak can sound a bit like "lifter noise" and it
makes sense that it would get louder under load, but not that the deep
clunk would go away quickly.  I suppose thermal expansion could seal it
somewhat...  Oh well, I am bringing it in and I guess we'll find out.
Cross your fingers. ;)