I have a 99 f-150 4.2 5speed. The past two months when i start up my truck i hear a rattling sound. One mechanic said it could be the u-joints but don't u-joints make a clunking sound when driving? This sound happens before i put the truck in gear and when i just start driving. I don't hear ir when i get up some speed. I thought it might be the catalytic converter.The exhaust doesn't feel loose. If it turns out to be the catalytic converter, is there such a thing as a reconditioned one. I know they are expensive. Any thoughts on what i should check. I can't tell if the sound is coming from the front or the back of the truck.

 

Check your engine hoist brackets. They're located on each side of the engine right beside the exhaust manifolds. Mine rattled up against the exhaust manifolds when I took off in low gear. I just tightened the bolts that hold them on, and bent them outwards away from the manifolds. They're flimsy, and will bend easy. You have the option of taking them off too. I didn't because if my engine ever needed to be pulled, I'd forget where I put the hoist brackets in my cluttered up garage.

Also check your motor mounts and tranny mounts. A loose heat shield will rattle too. Alot of times a small rock will get caught in between the converter and it's heat shield. There's a few things for you to check out. Good luck.

 

Often the heat sheild for the catalytic converter can rattle. Just tighten it up and the rattle goes away.

 

Thanks guys. I'll check those out.

 

It has to be ur Heat shields... I use to notice that noise when i was at a stop, with the A/C it'll make it even wors. ONce the the Cats where cut out. THE sound was no more, and it is now very quiet.

 

The muffler may have a loose baffle inside. if you tap on it with a rubber mallot you can find that.

 

As Peacemaker said check the hoist brackets. I had problems with mine in the past and they just started doing it again. Also, I have a very bad rattle with my exhaust because of broken welds. See if you can get the truck to rattle when in park with someone on the gas to see if you can pinpoint the rattle.

 

I actually just had this happen to me, I found that the exhaust pipe was loose right before the muffler and this caused the pipes to rattle.