I've got a '97 F-150 2WD Automatic with about 50,000 on it. Lately when I put it in gear and begin to accelerate it makes an awful rattling noise that sounds like its coming from underneath the truck approximately near my feet or further forward. This usually occurs once i get up around 20mph. If i just coast along it doesn't rattle, and likewise once up and going at 60 its fine. After I've gotten up to speed or things have warmed up, the rattle goes away. It won't rattle again until I let it sit turned off for a while.

It sounds kind of like a "tinny" rattle such as loose exhaust or a broken heat shield, but I've crawled under and checked everything for loose parts. The other thing that makes me think its transmission is that it has to be in gear. I can put in park or neutral and rev it without any noises.

Any ideas where to start?

Thanks

 

could be your cat. converter

 

I wondered about the cat, but I didnt see anything loose and that would rattle all the time wether in gear or not right?

 

My truck had a similiar rattle noise when idling in gear. It was actually the vent tube rattling against the side of the transmission.

 

Take a hammer or rubber mallet and bang lightly on the cat, and if it rattles then thats your chicken!

 

Sometimes the exhaust support bracket that is located between the transmission mount and the transmission tailhousing will crack and make noise. The crack can be hard to see, but sometimes you will see some rust along the crack's edge, usually on the left side of the bracket.

 

Check out you u-joints. They might needed to be replaced. Get the ones with grease zerks about $20 a piece.

 

Thanks for the ideas.

I took the truck down to the Ford garage and had them check it out. They had it for the last 2 days and supposedly a bunch of people tried to find the problem. What they finally came up with is the starter. The mechanic claims that the bendix isn't retracting all the way and it contacts the edge of the ring gear just slightly. They said it would need a new starter to fix the problem. I haven't had them order a new one just yet...

I have a background in electrical enginnering and as a hobby I've repaired AC motors, DC motors and even DC generators before. Anybody have any idea how hard a starter would be to tackle? It seems like it would just need to be cleaned / lubricated and maybe new springs or something?

 

That ain't a bad idea, except what may be wrong can't be fixed by a rebuild.
Go for it, but beware that the problem may require a new starter.