05 GMC Z-71 rattle??? Gear Heads input

[Ol' 30-30]

I have an 05 GMC 63,000 miles crew cab 5.3L Z-71 with a 6” suspension lift and 35x12.50s. When I drive over 50 MPH and hold the throttle steady it starts making this awful rattling sound and shakes a little. It sounds like a stick shift does when you are going to slow in 2nd or 3rd gear. It sounds like the noise is coming from behind the front seat (it’s not the engine). The transmission shifts fine and when I when it makes the rattle and I punch the gas it shifts fine and stops rattling. I replaced a transmission in my last Chevy and it doesn’t feel like the trans. It’s not leaking any fluids and the drive shaft isn’t loose or have any play when I push/pull on it. I have no idea what it could be. Rear end??? Any ideas??? Just FYI…It was over heating so I got the radiator flushed and a new thermostat last week, I don’t think this is related to the new issue. I’m officially stumped.

 

[andrew_dial]

check your u joints first. always start simple....and cheap! haha

 

[MYCAR47562]

do you feel any loss of power? sounds that it could be a clogged cat.

but if you wanna come to douglasville i will help you find the problem

 

[GoldDot40]

Could be a few things. Without experiencing it 1st hand, it'd be hard to say.

Look at your rear differential. If you see anything 'wet' looking around the yoke on the front (which is fairly common), it's possible that your pinion shaft bearing needs replacing.

Your driveshaft could be out of balance.

Your tires could be out of balance.

Could be a u-joint (as already mentioned)

It could be converter shudder......which might make sense being that it has overheated before. The trans fluid is routed through the tank on the radiator. When it overheated, it could have scorched the fluid. Has the transmission been serviced (fluid changed) since the overheating problem?

It's hard to say. It could be any one of the things already mentioned.....or NONE of them.

 

[Ol' 30-30]

I'm thinking it's my u-joints. No power loss and it's not running hot anymore. I'm going to look into more this weekend when I get some time. thanks for the input, I'll keep yall updated.

 

[andrew_dial]

oh and im assuming its a short bed but if its a long bed start with the midshaft bearing then replace the u joints.

 

[ParkerBorland]

Not sure if this helps but my 98 Z71 was doing the EXACT same thing you described so i carried it in and it was my torque converter. After replacing it, it quit. I am no expert at all, just thought i would share that. Good luck!

 

[bonecollector123]

check to make sure it's not in 4 wheel drive

 

[MYCAR47562]

You haven't figured out what it is yet?

 

[bilgerat]

mine does it too, its the heat deflector plate on the muffler rattling