exhaust rattle

[slow-truck]

Hoping my experience might help someone. Truck is 4.3 auto WT. Geared very high. When the torque converter locks up in 4th gear it will lug the engine at about 1100 to 1200 rpm. This caused vibration that resonates with a lot of stuff including the stock exhaust (it was probably wearing out inside, but looked fine outside). It sounded kind of like a Tuba. Replaced with a Gibson cat-back and that took care of that. However, there was still a rattle that occurred at exactly 1400 rpm as the engine vibration caused something to rattle under the hood. Thought it might be the O2 sensors? Nope. Replaced both driver's side with no change in rattle. Might be the internals of the Cats? Nope. Dealer replaced under warranty, but rattle was still there. Finally the mechanic noticed a small rectangular bracket riveted to the drivers side motor mount. It's about 5" x 1" and galvanized steel. One rivet had rusted out. It was resonant with the vibration at 1400 rpm. Dealer took it off the motor mount as it appeared to have no apparent function on this model. Maybe it's designed to keep stuff away from the mount, but there is nothing close to it. So for now, it seems I have a new truck. Guess I'll have to keep it awhile longer since it's got all new exhaust. It's really slow though!

[jblakeney]

Does the sound come from the front? Sounds a little like a loose converter material. .. ... If it sounds like its upfront, take a rubber hammer and bump the converter. If its loose it will rattle when you bump it too.