1997 Chevy FS pickup - SHUDDER

I own a 1997 Chevy 1500/Silverado/Z71, 3rd door, 4 X 4, 5.7 L., 104,500 miles - I love this truck. It runs and rides very well. I noticed on the interstate that if you punch it down around 40 - 60 mph
(wide open) the truck takes off like normal and sounds really good. But when you get up to around 3500 - 4000 rpm and 70 mph you suddenly here and feel like you're going over rumble strips. But louder and you can feel it in then seat (vibrations.) The sound, when you're sitting in the driver's seat, sounds like it is coming from the trans hump, in the front and up through the dash. Oil pressure, coolant temp. and voltage all stay in the normal positions. You cannot feel it in the steering wheel or gas pedal. The "Service Engine Light" does not come on. You can just hear it and feel it in your butt (through the seat.) If you back off the throttle, it stops immediately. But does it again 7 out of 10 times you put it through the motions that I described. If you keep the throttle mashed the noise will stop after about 6 - 7 seconds but I can't say that it stops when it shifts. Any advice or tips that anyone of you good people could provide would be appreciated. Doc, if you're reading this I would appreciate your opinion. Thanks
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If you mash it from a dead stop and redline through all the gears <pedal to the metal all the way from 0-70 mph> does it do it in any other gear but 3rd?
Doc

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check your u-joints
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No. No other gears other than 3rd. I tried this and it performed well up to that range.
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to
From your OP:
"If you keep the throttle mashed the noise will stop after about 6 - 7 seconds but I can't say that it stops when it shifts."
1) Is there a certain speed that it stops shuddering or just after 6 or 7 seconds? 2) You're saying that it doesn't stop when it shifts to 4th, or are you saying you're not sure if changing gears eliminates the shudder? 3) Last time she was tuned up?
Sorry for all the questions, want to have a good idea about the problem before I start guessing...............he he he he.
Hey G, you reading this? Sound like an AT problem? I'm thinking more motor mounts, AT mount or u-joints..............
Doc

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Well, I guess I just didn't know for sure. So I went out and tried it. Every time I tried it the shudder did disappear at the gear change. When the rpm dropped. 9 months since last tune up.
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Sounds like U-joints. Could also be transmission mount or motor mounts.
Doc

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Re: " I own a 1997 Chevy 1500/Silverado/Z71, 3rd door, 4 X 4, 5.7 L., 104,500 miles - I love this truck. It runs and rides very well. I noticed on the interstate that if you punch it down around 40 - 60 mph (wide open) the truck takes off like normal and sounds really good. But when you get up to around 3500 - 4000 rpm and 70 mph you suddenly here and feel like you're going over rumble strips. But louder and you can feel it in then seat (vibrations.) The sound, when you're sitting in the driver's seat, sounds like it is coming from the trans hump, in the front and up through the dash. Oil pressure, coolant temp. and voltage all stay in the normal positions. You cannot feel it in the steering wheel or gas pedal. The "Service Engine Light" does not come on. You can just hear it and feel it in your butt (through the seat.) If you back off the throttle, it stops immediately. But does it again 7 out of 10 times you put it through the motions that I described. If you keep the throttle mashed the noise will stop after about 6 - 7 seconds but I can't say that it stops when it shifts. Any advice or tips that anyone of you good people could provide would be appreciated. Doc, if you're reading this I would appreciate your opinion. Thanks"
-------------------------------------------------------------- How is the tranny fluid? Level,color,odor,last changed?
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Thanks to all for advice. Trans fluid level, color and odor are fine. I've only had truck for 1 year. Don't know when last change was.
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I'd make it so you know when the last change was. I do that whenever I buy a rig, doesn't matter to me if they said they changed it last week. Tranny, oil, power steering, differentials, t-case, brake... the list goes on & on....
~KJ/TLGM

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Thant's good advice. Thanks all!
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