buzzing under load under flooor

I am getting a sort of buzzing noise, that sounds like a heat shield rattling, it only happens when under load, and it seems to only happen at higher speeds (wont do it at idle or first or second gear), does not do it while coasting, and is almost constant when pulling a trailer down the highway, and about 50% of the time when unloaded, mostly up hills or when accelerating. The noise is definitely coming from the center of the truck and sounds like its right behind the cab. I have a regular cab short box truck, 5-speed. Would a bad center bearing or u-joints cause this? My center bearing and all u-joints feel tight, but my truck has 330,000KM on it and they are all original.

I had the same problem a few years ago. Turned out to be the Catalytic Converter heat shield rattling.

Might be your problem. If I remember correctly the factory one was a bolt on heat shield, and possibly a couple spot welds. get under the truck and hit the exhaust around the cat with a rubber mallet.

It is a possibility of it only making noise when the engine is warm because of heat expansion so I would hit it both hot and cold...

I dont have a cat convertor its been cut out and a straight pipe welded in.

well there goes that theory.

The only other heat shield I can think of is the one that attaches to the frame above the exhaust pipe (x-cabs might have 2), it is to shield the rising heat from the exhaust. I haven't removed it before so I'm not sure what is involved but you could check that???

I assume you have headers so the exhaust manifold heat shield is not there either

Other than that I don't know what it is. Just get under there and smack everything with a mallet. haha

Good luck.