I just bought a used 06 T & C van-97K miles. Had it about a month. Has power locks and both side doors open/close remotely.
I noticed the passenger side sliding door makes a buzzing noise then the locks automatically lock when I reach the 18 mph speed.
Any ideas what causes this buzzing sound?

 

Both rear doors auto-lock when moving. Depending on the 'country code' ie America or United Kingdom. They can be variously set different[ly] by a code reader for example (1) lock over 18MPH but allow unlock while stationary in PARK (2) never allow the rear doors to unlock under any circumstances unless the engine is turned off. It will I assume almost certainly be different for different years [software revisions].