I have noticed my truck (97 f150 4.2l 5spd) knocking for almost a 18 months. Took it to 3 different mechanics and all said don't worry its spark knock. It only did it under load like pulling my enclosed trailer, or when I put the pedal to the floor in a higher gear... Well then about a month ago I heard what I assume was a cat vibrating when I started or switched gears. Well this past weekend I was driving home with my trailer and I noticed the fan was running like it does when the AC is on (engaging for the first 2 gears then disengaging), but the AC wasn't on. Well about 5 miles into the trip I start to loose power and the truck is almost feeling like its misfiring or somthing similar to it. Temp is great, but I pull over and shut it down. I have a buddy get the trailer, and let the truck sit until that night. I came back and the truck started up and ran fine but something is rattleing bad underneath once in a while. I had a cat come apart in my 97 tbird and the car just shut off and wouldn't run... Could this be a bad cat just getting lodged in the exhaust and causing backup and excessive heat or something?

 

Sure.
Now what would you do to find out?
Hint******* measure back pressure or remove the cat assembly for a trial run.

 

Sure sounds like it could be a clogged cat.

FWIW, back in the day my mom had a late 70's Ford Wagon, and the cat went bad on it. When the car was cold (like overnight) it would start and run fine, and it would be able to hold highway speeds even when hot. But try to climb a grade on the highway with the motor hot, and it would barely make it over, driving in the emergency lane at 10 mph with the hazards blinking. A cat 'test tube' replacement and it ran as good as ever.

 

Muffler shops have infrared testers that can detect clogged cats without opening anything up. It can also be collapsed baffles in the muffler or a collapsed exhaust pipe.

 

are you sure glc isn't cramming taters in your tailpipe?

 

That was dumb.

Why would you NOT worry about spark knock?

Cat converters don't just go bad. That's the problem.

Running a vehicle that is dumping unburned fuel into the exhaust that eventually nukes the converters and possibly has ruined the engine is bad.

So, to answer your question...the sky is the limit on how bad the damage could be.