Machine gun sounds at speed

[gregmajor]

I’ve got a 1979 K10 full-time 4WD that I’m getting back on the road. 350/TH350/NP203 combo with new front axles, seals, and such. New axle seals all around in the rear, too. The driveshaft was rebuilt as well.

Where I’m presently stuck is a noise that shows up at 40-45 MPH from under the truck. It’s a sort of “machine gun” sound (though not nearly as loud). More M16 than A10 Warthog in terms of speed if you get me.

It doesn’t seem to change with speed and always goes away once I back down below about 40. I haven’t tried pushing beyond 45 for fear of doing some real damage.

Anybody have suggestions on how to begin troubleshooting?

Thanks in advance!

 

[PrairieDrifter]

On my 79 with same drivetrain, I had some tires that started coming apart internally, I would say it was pretty machine gun like. It would go through the drivetrain and rattle the transfer case shifter. After the tire let go it never did it again.

Was almost kind of pulsating too. Right in the 40-55 range.

 

[Snoots]

Make sure some clown didn't put zip ties on your driveshaft.

 

[gregmajor]

Turns out the exhaust header flange is just barely making contact with the balancing pad on the front driveshaft. I may not have caught it except for I just happened to see it when diagnosing a fuel line leak.

 

[SquareRoot]

Turns out the exhaust header flange is just barely making contact with the balancing pad on the front driveshaft. I may not have caught it except for I just happened to see it when diagnosing a fuel line leak.

Glad you found it. Most people don't realize that driveshaft angle changes under load. At 40 mph the drivetrain is under load just from the wind resistance. That's why it's usually set at about 3 degrees static and moves close to zero while driving. And before someone chimes in, driveshaft angle is not the same as u- joint angle.