[str8.ght acceleration]

There is a TSB for the lower door sill plate rattling at 45-50mph on rough roads... The fix is to install foam in the sill and secure..
Might me your problem..

TSB 23-005-06

Symptom:
A rattle can be heard at speeds of 40-50mph at higher temperatures 80-90 degrees, especially during cross wind conditions. The sound may be mistaken for a lower door rattle. The rattle sound is caused by contact between sill cladding flange and lower body pinch weld.

Repair takes less then 1/2 hour..