I recently swapped a 5.3/4l60e into my 1992 GMC truck. Drove it about 400 miles and started hearing what sounded like my Walbro fuel pump that I have mounted on the frame rail but louder than it had been previously. Looked under the truck for all of 15 seconds and dismissed it as the fuel pump and assumed I just hadn't heard it yet because I was focused on other sounds and looking for bugs. At this point I wasn't really getting into the throttle very hard, at least not often. Over the next 100 or so miles I noticed it getting louder and louder and my gut reaction was I probably should change my fuel filter. That same day I was heading home from work and went to pass on a 2 lane road. Rolled into the throttle, the truck downshifted into 2nd and the RPM's hit 4000 and wouldn't go any higher. I got out of it and babied it home. Put it in park and revved it clean to 6000 no problem. Thoughts of the fuel filter and fuel starvation under load came to mind so I changed it. Went back out to see if that solved the problem, hit 3500 RPM and couldn't go any higher. Ok, so I got a bad junkyard trans. Bought a rebuilt, upgraded unit and swapped it in along with a 2800 stall converter. Low and behold about 200 miles later I've got the EXACT same issue with the new trans. I've got a video of the noise but I still need to figure out how to post it. It sounds just like a fuel pump or a hair dryer. The noise isn't RPM or gear dependent. Happens in park/neutral and in every gear. I can be at idle or 3000 but the sound never changes, constantly the same hum. Anyone have any suggestions? I'm talking with a few different reputable local shops, guys I know, but nobody can seem to give me an answer.

 

Something solid from the floor board or somewhere is touching the transmission case.
A screw, the dipstick, etc.
Maybe something on the firewall. Could be something from the back of the engine touching the firewall.

 

Seems like the sound would radiate through the cab then too. It's just from under the truck. Also, I don't see how the case touching another part of the vehicle would cause the trans to slip.

 

Oh so the new transmission slips too?

 

Yes both the junkyard trans and new trans slip. I put in a new converter with the new trans. It's something external causing an issue in the trans. My thought is something with the wiring harness, however I'm certain I never touched any of the plugs to the trans. But I'll tone it out as soon as I get a chance and see if I can find anything. Any other suggestions would be great too!

 

Heres a video of the sound it makes.

 

Nobody has any suggestions? Bummer.

 

I do.
If it doesn't change with rpm or speed it's not the transmission.
A pressure test might confirm this to be sure it's not some weird thing that causes valve buzzing. Still I can't think of anything transmission related that doesn't change at all with rpm or mph.

And even "a few reputable shops" have no clue and they have more than a paragraph and video, they have the actual vehicle I'm assuming.