Vibration over 70 please help!

[SamJam05]

Good morning,
I have a vibration in the center of my 88 Monte over 70 mph. It’s more of a whirr sound but you can really feel it with more mph. It does not go away over 80 when you let off, but goes away 75 mph ish when you let off. Thing I’ve done to combat it:
New driveshaft/ujoints balanced at shop. New rear brakes/drums. New tail shaft bushing. Rear end is freshly rebuilt (all new parts from ring to pinion) with less than 1,000 miles. No rear end howl, sounds good on lift. All new front suspension, 2 inch drop with new ball joints. Drop using spindles and fresh alignment. Tires were mounted and balanced last year. 0 vibration last year before the stock rear gear decided to explode. I’m out of ideas, pattern looks good in rear, all within gm tolerance spec. I’m sure there’s more ask questions if needed, but sure could use some help, been chasing it for months.

It’s LS swapped with a th350 with BMR rear upper control arms and the pinion angle is perfect. Trans points down 2 degrees, driveshaft is at 0 and pinion is up 2 degrees. I’ve changed the pinion angle several times with no noticeable difference. Any thoughts?

 

[popeye1978]

Do you feel anything in the steering wheel when this vibration occurs?

I'm a bit unclear on the speed conditions so check the following statements:

• If you were on an infinitely-long highway and at one end you started from a dead-stop, this vibration doesn't start until you hit 70
• You have found that if you reach 75 and then let the car coast, the vibration disappears; if you reapply gas above 70 does the vibration come back? What if you reapplied gas at 65?
• You have found that if you reach 80 and then let the car coast, the vibration remains; what happens if you let it coast below 75?

 

[Built6spdMCSS]

Check the Torque Convertor and Flexplate?

 

[SamJam05]

Do you feel anything in the steering wheel when this vibration occurs?

I'm a bit unclear on the speed conditions so check the following statements:

• If you were on an infinitely-long highway and at one end you started from a dead-stop, this vibration doesn't start until you hit 70
• You have found that if you reach 75 and then let the car coast, the vibration disappears; if you reapply gas above 70 does the vibration come back? What if you reapplied gas at 65?
• You have found that if you reach 80 and then let the car coast, the vibration remains; what happens if you let it coast below 75?

I don’t feel anything additional from steering wheel, all vibration is under the butt. Asked passenger to feel it and they also believe it’s centric.

Basically, if I floor it, it vibrates a little over 40 ish mph (I know I’m giving additional info now just trying to remember every detail) then goes away with no throttle. But at cruising throttle there is 0 vibration until just around 70. It comes on around 75 and if I coast it goes away immediately (even if coasting and remaining at 75). I stated this before because it’s definitely worse under a load then at cruise up to about 80. After 80 it’s always there. If I reapply after 65 say at light throttle it’s not there but definitely there under a good load. The vibration does tend to go away around 75 if I let it coast back down from any speed above. I hope this makes sense.

 

[SamJam05]

Check the Torque Convertor and Flexplate?

Flex plate is the same as before appears completely fine, had no vibration before. It’s a different torque converter than before the rear end blew but came out of a similar set up car with no issues whatsoever. I will look into those anyway on a lift, definitely a good call thank you.

 

[Built6spdMCSS]

The only other thing left is axles..

 

[SamJam05]

The only other thing left is axles..

I imagine it’s possible the twisted when the spider gear went? Maybe bound up?

 

[86LK]

so basically, only happens under load and at high speed? goes away when coasting at high speed?
what happens if you manually shift to the next lower gear (4th to 3rd) at high speed before you hear it? does it start noise?
what happens if you are in a lower gear at high speed making noise and then shift up (3rd to 4th)? does the noise go away?

 

[78Delta88]

Hard to say just by description, but as a young budding mechanic ... I broke more than I fixed, until I realized that's a bad business model and got my self into classes, and hung out with the older folks that knew way more than me.

Low to medium pitch, gaining pitch under load and speed is typically pinion to ring mesh. It might sound great on a lift but there is no load of the car weight on the bearings and etc. It is possible it is fine as is, but changes once vehicle is on its full curb weight and moving forward shows it could be bearings or other issues in the setup.

You did thorough work on diagnostics of the noise so balance of the peripherals is probably not it.

Things changed were TQ so is possibility easy to rule out but will have to pull trans to swap out.

Main two things I see is differential noise, or internal in trans.

To me and based on experience, it sounds like a differential setup issue, yet could also be slop in the trans. The internal total run-out is controlled by selective nylon washers and not a lot today understand how to setup an older TH350 or TH2004R/700R4. Engine RPM at 3,000 can easily have internal components at 20,000 RPM or more. This can produce the issue you are having.

Then also have to ask... Did you try rotate tires? Did tread style change? Tire with slipped belt?

 

[78Delta88]

Just rereading though this again..

You say you can "feel" this and increases as speed increases...

It could still be differential, but more inclined to think transmission.

Mainly you can "feel" it and that seems to point to excess slop in a rotating item. TH350 have noted Drum problems, and I have rebuilt a few where the matting surface of the Forward Drum was completely worn away.

If you have ruled out the other things, I would pull that trans and pull apart.