Hi everyone,

I’m looking for some advice regarding a vibration issue that started after recent work on my 1971 Oldsmobile 442 (TH400 automatic).

Here’s the full context:

A few days ago I brought the car to my mechanic for several jobs:
- Water pump replacement
- Fan clutch replacement
- Radiator replacement
- Thermostat replacement (180°F)
- Coolant outlet neck replacement
- Power steering pump seals replacement
- Engine oil and filter change
- Fixing a leaking exhaust manifold (co-driver side)

Additionally, I wanted to replace the rear main seal. To do that, the transmission, torque converter and driveshaft were removed. However, once everything was apart, we realized that accessing the rear main seal would require removing the crankshaft, so we decided not to proceed and reassembled everything exactly as it was.

Important note:
The torque converter was not drained separately and was reinstalled together with the transmission. The driveshaft was also reinstalled, and according to the mechanic, it was placed back in its original position.

The problem

Since getting the car back, I have noticed the following:
- A noticeable vibration and humming noise starting around 50 mph (~80 km/h)
- The vibration is felt throughout the whole cabin (not just steering wheel)
- It gets more noticeable as speed increases
- The car initially felt sluggish and the transmission was behaving erratically

After checking, I found the transmission fluid level was incorrect (likely due to radiator replacement and cooler lines). I corrected the ATF level properly (hot, engine running), and now:
- The transmission shifts correctly again
- The car no longer feels “lazy”

However:
- The vibration is still present exactly the same as before.

Key observation (very important)

At around 50–60 mph, when the vibration is present:
- If I shift the transmission into Neutral (N) while rolling, the vibration immediately disappears.
- When I go back into Drive, the vibration returns.

What I suspect so far

Given this behavior, I believe the issue is related to something that only happens under load, possibly:
- Torque converter issue or misalignment
- Driveshaft (U-joints or angle issue)
- Transmission output shaft / yoke
- Something related to how the transmission was reinstalled

Before this work, the car had no vibration at all.

Questions for the experts:
- Have you seen similar behavior on a TH400 after removal/reinstallation?
- Could a torque converter that “seems fine” still cause this type of vibration?
- Is it possible for U-joints or driveshaft angle to cause vibration ONLY under load but not in Neutral?
- Any known quirks on these cars regarding transmission alignment or crossmember positioning?
- Could an exhaust component (especially after working on the driver-side manifold) be touching the frame or transmission and cause vibration only under load, disappearing in Neutral?

Any insight or similar experiences would be greatly appreciated. I’d like to narrow this down before taking the car apart again.

Thanks in advance!

 

Start by looking the exhaust over, make sure it’s not touching anything.

Try flipping the driveshaft 180 degrees. You might try reclocking the converter.

Inspect the engine mounts, and the transmission mount.

 

X2 ^^^

Check the universal joint movement is it notchy, is a needle missing? Driveshaft angle and pinion angle can cause exactly what you describe, examine the transmission mount and its installation for issues.

 

While you're under the car examining "stuff" locate exactly where the mechanic put his scribe/location mark(s) on the entire driveshaft assembly to "confirm" the drive train assembly was re-assembled exactly as it was dis-assembled. If there are no location marks....well.

 

How about checking the torque on everything that was disassembled.
Being as it was a no go on the main seal things may have been hastily slapped back together and something didn't get tightened.

 

I would check the rear u-joint. The caps can easily fall off that one when the driveshaft is removed. The front is contained by the driveshaft and the transmission yoke, so it can’t “fall apart” during driveshaft removal.

 

Also check all of the bell housing bolts (6).

 

Any update? I am very curious.

 

Careful, Greg. Remember what happened to the cat?

 

He probably did what I did and forgot the fan clutch shim. put it into first and go for a drive and see if it does it at that rpm, same at second.

 

Hi guys.

Just to clear things up: I rotated the driveshaft 180° and the problem was solved.
I do notice a slight vibration around 70 mph, but it’s very mild. It’ll probably disappear after torquing all the bolts to spec to get everything perfectly aligned.
Thanks a lot for all your advice and recommendations — it really helped me track down the issue easily.

Regards.

 

Exactly the reason why I asked to check the location of the mechanic's witness (scribe) marks when he removed & replaced the propeller (drive) shaft. The mechanic either lied to you or doesn't know what he's doing. In both cases, I'd find a different (more worthy) mechanic.

 

Consider getting the driveshaft checked for straightness and balance to get rid of the rest of the vibration, it will make a noticeable difference.