I have a 2004 Ford F150 FX4 5.4L that sounds like a diesel. Every time I leave the engine running and someone walks by they stare at it because of the noise and I often receive comments from people in the know that it sounds like a diesel pickup.
Exhaust noise is normal but there is a distinct clicking noise from the engine that makes it sound exactly like a diesel pickup. The only time it doesn't make this noise is occasionally, but not every time, at initial startup to within an hour of run time on a cold morning. Truck currently has 63K miles and has made the noise since I purchased two years ago with 34K. No other problems noticeable.