Here's a tad of info from a previous post...
ISSUE:
2008 F-Series vehicles equipped with the 6.4L diesel engine are equipped with an oxidation catalytic converter (OC) and diesel particulate filter (DPF). The function and operation of these parts may cause some customers to perceive an issue with their vehicle.

ACTION:
Review the Operating Characteristics with the customer.

OPERATING CHARACTERISTICS

Diesel particulates in the exhaust are trapped by the DPF. Regeneration is the process by which exhaust temperatures are increased so the particulates are combusted.

The frequency and length of regeneration will fluctuate as both are determined by the drive cycle. For most drive conditions, regeneration frequency will vary from 100 - 600 miles (161 - 804 Km) between occurrence and last from 10 to 40 minutes. The first regeneration does not require 100 miles (161 Km) and may occur at any time. The length of regeneration is usually reduced if a constant speed above 30 MPH (48 Km/h) is maintained.

The following is a list of normal operation while the vehicle is in regeneration, and do not require repair. If you are not sure if the vehicle is in regeneration, IDS can be used to monitor the Diesel Particulate Filter (DPF) PID.

Engine idle speed can be 1100 to 1200 RPM in park/neutral with foot off brake.
High idle speed drops to within 50 RPM of normal idle when the brake pedal is touched, PRNDL is actuated, or clutch is actuated.
White smoke in cold ambients is normal and the amount will be increased during regeneration.
Powertrain power is limited to 325 horsepower (HP).
Engine responsiveness may be slightly different than normal operation.
During initiation of regeneration, exhaust smell may be noticed - especially on new vehicles.
Powertrain sound will be different including air induction noise (including flutter on deceleration or engine shut down), exhaust noise, and changes in engine radiated noise.
During regeneration, exhaust temperatures are elevated.
The following is also normal and may be observed by a technician using a diagnostic tool. It is not likely that a customer would be aware of these:

The throttle body is only active during the regeneration process and during shutdown.
EGR is not operating during regeneration.