I was looking at a used 2001 F-350 Lariat w/7.3 dsl, 4WD, automatic today w/ ~70,000 mi. Unlike my 2001- 7.3, it has a low frequency rumble/vibration at idle that is a bit un-nerving. It has a stock exhaust system and I cannot find any clear reason for the difference between my truck, which is virtually identical right down to paint scheme to this one. My fear is a transmission or converter problem. Any ideas?