Bought a 2003 f150 supercab STX, 4.2 6 cylinder last month. 37k miles. Noticed a sound in it day after I bought it. Sounds like a vibration, but not felt. Is loudest in 3rd and 4th gear, and only when there is a load on it. (like when accelerating or maintaining on a slight hill). Asked a salesman at the Ford dealer where I bought it and he said catlytic converter. Let the service people look at it and they said ball joints. They replaced the lower ball joints. (they paid as they say they will stand behind their used vehicles). Sound is still there. Took it back and they said they drained the tranny, scoped it, refilled it, talked with Ford half the day, and nothing wrong with the truck. Noise is still there. They now say it is normal, but they don't know what it is. (seems to me they should know what it is if it is normal). Noise is coming from tranny or under passenger floor board. They say it is noticable because I have the plastic floor, not carpet. I can't really dispute Ford, but it doesn't sound right to me. Any one out there with the same engine and noise? Is it normal? Any ideas?