It's starts as a soft tapping coming from underneath the vehicle, and it is somewhat intermittent. Soon though that clanging noise occurs at all speeds and all engine operating conditions. It seems to get especially loud when you go through the drive-through window at your local fast food joint. Don't get scared - even though it sounds as if it could be coming from your transmission, the noise is most likely just the shield on your catalytic converter.
The shield is spot welded together around the cat like a clamshell. Typically, two welds on the bottom (towards the rear of the vehicle) will fail, allowing the shield to bang against the catalytic converter as it vibrates.
There are three (well really four) levels of solution to this problem:
I wish to update your listing on catalytic converter rattles. The part (a catalytic converter strap) is available at your Ford dealer. From the Ford service manual the number is: 383735-S. Check with the service dept first as it might be covered under the emissions warranty. FOTZ-5A231-A
Jeff's note (from Tom Wolf) - this part is really just a Ford "branded" 7 inch stainless steel hose clamp. The actual Ford part number is FOTZ-5A231-A and the cost is about $5.00 plus tax.
On my 93 Explorer 2WD, I was able to secure broken attachment points of shields with blobs of high temperature RTV silicone. As the metal "tabs" broke, I continued to fix them with RTV. This process worked for a year or two at which time the exhaust pipe had to be replaced due to rust through. Don't know how long it could have lasted. It's best to catch these broken attachments early, because if left unattended, resultant increased metal fatigue on remaining attachment points will cause them to break more quickly. There's not much mechanical load on the shield attachment points, and the silicone seemed to make a pretty strong bond without any special surface prep.
When my 92 Ranger started making noises from underneath, I took it to my local Meineke place. They diagnosed it and spot welded the shield for nothing. It has stayed fixed for five years now.
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